I would highly recommend the gmatpill
RC Pill Strategy.What you're doing is spot on in theory -- you're just missing the application.
Yes, you do want to read the first sentence carefully -- but it's not always the case that every RC Passage will tell you what the whole passage is about from just the first sentence. Sometimes it may take a few sentences before the reader can finally decipher what direction the author is taking on the topic at hand.
It's about thinking critically while you read. You did pay attention to transitive words and you say you try to understand what you're reading...but certainly the order in which you do things is going to affect your comprehension. And when looking at an RC question, you are reviewing the text perhaps too long. This suggests the entire process of "scanning" without any clear purpose is going to slow you down and cause you to get questions wrong.
Do what you're doing but do it with greater purpose. At GMATPill, we recommend you to figure out the main idea and the "skeleton" of what's going on with the passage. Then once you get "main idea" questions you should be able to answer them quickly. Upon getting "detail" questions, you should be able to jump to the right spots in the passage (sometimes multiple), in order to zone in on getting the correct answer.
Hope that helps.