Surfing Waves

I've been surfing for almost a year on 5'11 shortboard. I have this problem with my frontside top turns, most of the times when I go down the line and see a pocket I want to hit, I do a bottom turn and when I go up the face and try to top turn I fall backwards but when I try to put my weight more on the front - the wave seems stronger than my leg and I just can't push hard enough and my the wave just hits the board out of my legs.
Any ideas why it could happen?

Just read through your posts and you've still got the problem most shortboarders have in their first year , that of generating speed, when you fall backwards at the top of the wave for a top tur you haven't genreated enough speed to allow you to maintain that balance you need speed is the essence of every turn you do, without it the board bogs, digs a rail and or just drifts over the back of the wave.
If you can get a friend to video you surfing watch yourself, the harshest critic will be you, but you can learn a lot. Watch the good guys at your beach see how they generate power and speed.
BTW speed also add postural strength to your legs!

what size fins you got on?