Here is some advice you can follow to improve in RC and SC. I’ll start with Reading Comprehension.
To improve in Reading Comprehension, you need to focus on understanding what you are reading. When you incorrectly answer Reading Comprehension questions, it’s partly because you didn’t truly understand what you read, right? Thus, you likely have to slow down in order to (eventually) speed up. At this point, your best bet is to focus on getting the correct answers to questions, taking as much time as you need to see key details and understand the logic of what you are reading. You have to learn to comprehend what you read, keep it all straight, and use what you are reading to arrive at correct answers. If you don't understand something, go back and read it one sentence at a time, even one word at a time, not moving on until you understand what you have just read. There is no way around this work. Your goal should be to take all the time you need to understand exactly what is being said and arrive at the correct answer. If you can learn to get answers taking your time, you can learn to speed up. Answering questions is like any task: The more times you do it carefully and successfully, the faster you become at doing it carefully and successfully.
Another component of understanding what you are reading is being “present” when reading. Don’t worry about how things are going at work, or what you will eat for dinner, or even how long you are taking to read through the passage. Just focus on what is in front of you, word by word, line by line. Furthermore, try to make reading fun. For example, even if you are reading about a topic that bores you, pretend that you are the person making the argument. By doing so, you will make the passage more relatable to YOU, and ultimately you should be able to read with greater focus.
One final component of Reading Comprehension that may be tripping you up is that RC questions contain one or more trap answers that seem to answer the question but don't really. So, a key part of training to correctly answer RC questions is learning to notice the differences between trap answers and correct answers. You have to learn to see how trap answers seem to follow from what the passages say, but don't really, while correct answers fit what the passages say exactly.
Regarding SC, there are three aspects to getting correct answers to GMAT Sentence Correction questions: what you know, such as grammar rules, what you see, such as violations of grammar rules and the logic of sentence structure, and what you do, such as carefully considering each answer choice in the context of the non-underlined portion of the sentence. To drive up your Sentence Correction score, you likely will have to work on all three of those aspects. Furthermore, the likely reason that your Sentence Correction performance has not improved is that you have not been working on all three of those aspects.
Regarding what you know, to be successful in Sentence Correction, first and foremost, you MUST know your grammar rules. Let's be clear, though: GMAT Sentence Correction is not just a test of knowledge of grammar rules. The reason for learning grammar rules is so that you can determine what sentences convey and whether sentences are well-constructed. In fact, in many cases, incorrect answers to Sentence Correction questions are grammatically flawless. Thus, often your task is to use your knowledge of grammar rules to determine which answer choice creates the most logical sentence meaning and structure.
This determination of whether sentences are well-constructed and logical is the second aspect of finding correct answers to Sentence Correction questions, what you see. To develop this skill, you probably have to slow way down. You won't develop this skill by spending less than two minutes per question. For a while, anyway, you have to spend time with each question, maybe even ten or fifteen minutes on one question sometimes, analyzing every answer choice until you see the details that you have to see in order to choose the correct answer. As you go through the answer choices, consider the meaning conveyed by each version of the sentence. Does the meaning make sense? Even if you can tell what the version is SUPPOSED to convey, does the version really convey that meaning? Is there a verb to go with the subject? Do all pronouns clearly refer to nouns? By slowing way down and looking for these details, you learn to see what you have to see in order to clearly understand which answer to a Sentence Correction question is correct.
There is only one correct answer to any Sentence Correction question, there are clear reasons why that choice is correct and the others are not, and none of those reasons are that the correct version simply "sounds right." In fact, the correct version often sounds a little off at first. That correct answers may sound a little off is not surprising. If the correct answers were always the ones that sounded right, then most people most of the time would get Sentence Correction questions correct, without really knowing why the wrong answers were wrong and the correct answers were correct. So, you have to go beyond choosing what "sounds right" and learn to clearly see the logical reasons why one choice is better than all of the others.
As for the third aspect of getting Sentence Correction questions correct, what you do, the main thing you have to do is be very careful. You have to make sure that you are truly considering the structures of sentences and the meanings conveyed rather than allowing yourself to be tricked into choosing trap answers that sound right but don't convey meanings that make sense. You also have to make sure that you put some real energy into finding the correct answers. Finding the correct answer to a Sentence Correction question may take bouncing from choice to choice repeatedly until you start to see the differences between the choices that make all choices wrong except for one. Often, when you first look at the choices, only one or two seem obviously incorrect. Getting the right answers takes a certain work ethic. You have to put in the necessary time to see the differences between answers and to figure out the precise reasons that one choice is correct.
To improve what you do when you answer Sentence Correction questions, seek to become aware of how you are going about answering them. Are you being careful and looking for logic and details, or are you quickly eliminating choices that sound a little off and then choosing the best of the rest? If you choose an incorrect answer, consider what you did to arrive at that answer and what you could do differently to arrive at correct answers more consistently. Furthermore, see how many questions you can get correct in a row as you practice. If you break your streak by missing one, consider what you could do differently to extend your streak.