Most of the trick is done with your back foot. You pop with the back foot, and catch the board, level it out - your back foot and leg are the star of the show. Keep practicing, but here are some common problems people face when learning how to No Comply:
Kicking the Board Out - Sometimes, skaters have a hard time where they go for the No Comply, but end up kicking the board out in front of them and losing it. This is usually a problem with not getting the scoop in with the back foot. You don't want to just pop the tail and then hope for the best - you want to get the scoop in so that the board pops up against your leg, all in a controlled way.
Too Low - This is a very common problem, and happens because of a lack of commitment. You need to make sure you really crouch down and give this trick a jump. Not just a little hop.
Losing the Front Foot - If you have a hard time getting your front foot back on the board, this could be because you are scared, or because you don't have much control. Either way, take it slow and practice some more. It'll come.
Keep practicing, and get creative! The No Comply is one of the few skateboard tricks that, like the Boneless, let you put one of your feet on the ground. This opens the doors for all kinds of inventiveness! Have fun!