Try to place your front foot more towards the front of the board (about 1 inch before the bolts) and more in the middle.
Then make sure you are timeing it right, pop then flick, in a diagonal motion, so that your foot comes off the edge of the board diagonally. Dont kick your whole leg out just flick with the ankle and leg a tiny bit. And make sure your can do other stuff like ollie of course

. Also try to learn varial flips first they are easier believe it or not
www.skateboard-city.com - has some good trick tips (with videos)
just keep trying, you should get it.
hope i helped