Do you have any explanation? Did you mess up with your pacing? Were you overly nervous? Did you never write the essays on your practice tests, and now had to write them for the first time?

The GMAT Prep scores tend to be very good representations of true ability, especially considering that you got the same score on both of them. I think that with a little more studying - and maybe a few more practice tests in simulated test conditions - you should be able to get up to that level.