You'll need to figure out why you got them wrong the first time but could do them the second time. There could be different reasons for different problems—maybe your process is not consistent enough for a certain question type, or you forgot a certain grammar rule that you remembered later.
There could also be an overarching reason why this is happening for the entire section.
For instance, if you are spending too long on certain questions and then having to rush on other questions to catch up, then that may be why you are missing some questions the first time but answering them correctly later, when you don't have the same time pressure. You'd likely think of these as careless mistakes when you were reviewing the problem.
Alternatively, if you are experiencing mental fatigue or anxiety during the test, then you would make more mistakes (especially in verbal, the last section of the test), even though you really do know how to do those problems. Mental fatigue generally feels like you just can't think properly / clearly, you have to re-read things that you don't normally have to re-read, etc. Or mental fatigue can also result in you rushing because you just want to be done with the test. Or you keep agonizing back and forth between 2-3 answers and can't make a decision.
Anxiety can also distract you / make it difficult to concentrate. For mental fatigue and anxiety, I recommend this:
https://www.manhattanprep.com/gmat/blog ... mat-score/or this:
http://www.10percenthappier.com/mindful ... he-basics/Do you think any of these situations apply to you?