DS is a question type which we are not used to solving. I feel that the more DS problems you solve, the more familiar you will get with the question type.
As far as SC is concerned, there are 2 stages:
i> awareness of the concepts - There are no short cuts here. You have to go through them repeatedly till you are familiar with them. If you stop doing SC for some time, you might feel some concepts have become hazy for you. At that point pick up SC theory and start again.
ii> application - knowing the rules alone don't guarantee good performance in tests. If you knew all the rules of cricket, it would not make you a great cricketer
Practice is essential, along with noting every error and cause of errors. Also go through the ones you got right. Make sure you can explain your decision for accepting or rejecting an answer option.
There are 2 other hurdles in GMAT: Fatigue and time pressure. Do you think these could also be the reasons for your scores not going above 530?
What is your target score?