Why cant I ollie on a skateboard and what am I doing wrong?

as some of the other answers say, you aren't popping hard enough, a good way of imagining it is as if you are quickly putting your hand over a fire just the same way as when you jump you quickly kick the tail which makes a pop, so really you want to jump high and with force as well as kicking the tail but remember not to stomp on the tail because then your board goes no where, here is a good video that i used to give me the right idea: http://www.youtube.com/watch?v=MTp5_jSPJE4