As athoul says just keep practising. But it isnt just an abiltiy thing ( we are just talking jumping no big thing in itself). There is a mental thing. Now you will see vids on here of people doing just amazing things. Just cos you ( and me) cant do them does not make us freaks wimps or failures. Some can just go jump of a 10 foot wall jump up 6 or 7 steps. Others just cant, the brain doesnt let them. You have to prove to your head you can do it.
I met up with a mate yesterday. He has stolen a march on me and has done 'flips' , well 2 onto matts to be exact. But he cant 'jump up'. Now im not good but he was balking at 1 step ---but he can flip goddamn it!!! He can do amazing things but not simple things.
So what to do, well you know jumping is simple ( tho u cant do it at the mo). So go look at the vids and pix here, brake them down, then go practice each bit. Best if you have a fellow projumper with you , but failing video yourself if you can or jump infront of shop windows