If your friend isn't stable then he's got some issues to work out!
But for real, just keep working at it. Some people just have a knack at popping. I play volleyball and skateboard, I can jump pretty high and I pretty consistantly get more height and distance off jumps then the adverage riders that are hitting them. To me there is a lot more to getting height then just jumping at the lip. How you you load up, and when are huge factors. For example, if you jump from a bent position you wont jump as high as you would if you bend down and jump all in one smooth motion. Also make sure your jumping off the balls of your feet, not flat footed.
Anyways, I'd rather see a rider jump low and smooth then high and flailing, just keep working on your timing and slowly popping harder. Try it everywhere as well, not just off the jumps. If thats not doing it then start doing squats and pliometrics. It could just be that your buddy can jump way higher then you.