Can you get a video clip of you doing a wheelie? It might help a bit...

http://jonbuder.com/videos/Biking.wmv - near the end are some examples of UNsuccessful wheelies. That was back when I got the bike, I just basically pedalled hard and hoped for the best.

Really helps to lean back far. In fact, kinda throw your weight back if possible, quickly. Try standing up on the pedals and see how far you can pop the front wheel up. If you can get the front wheel all the way up and jump back off from a standstill, I don't get what the problem with wheelies is.