Kaplan is not the best way to go...their stuff can be so misleading in my opinion. Are you using the OG stuff? OG stuff is best way to go and your GmatPrep score seems to be close.

Kaplan is just too tough usually.

I think you should work on your basics a little bit and then practice a lot of questions. You can work through both editions of the OG i.e. OG 10 and OG11. For brushing up your fundamentals I suggest that you work through the Barron's GRE book. Yes i mean the GRE book by Barron's not the GMAT. It has a great math fundamentals review and then you can practice tons and tons of questions. GMAT official guide has more than 400 questions which you can practice from. With good basics and improving fundamentals you should be able to cross the 600 barrier.

however you must be willing to put in hard work. I know that it is not easy to do that but it is the key to success. Good old fashioned hard work

I hope this helps.

All the best.