My best recommendation is to start trying to answer questions without doing the math... That may sound a little strange, but when you start to try and estimate answers, look at answer choices for guidance, and then eliminate wrong answers, you will start to see that you can cut down on the time each problem takes and get to a place where you can make an educated guess more quickly.

Look at Problem Solving question number 16 in the Official Guide 13th edition.

How can this problem be solved with one simple calculation?

I want to add a caution to Jim's advice, and that is that guessing strategies are to be handled with care. There is no way the GMAT is ruling out an answer just because its the only odd one, or the only one in single digits, or the only one not showing in the question itself. You can rule even answers because you deduce that the answer must be odd, but that deduction cannot be made without understanding the question pretty well.

You do say there are no problems with your math. That's where I would look for new points. You may be calculating accurately, but inefficiently. In such cases it would look as if there was no problem with your math.

Kind Regards,

Misterholmes

It is sort of my point that you have to understand the topic well to be able to work through with less math. If you can push yourself to that place, then you will have a greater understanding of the topic.