Read the "Can't Bunnyhop" thread in the Beginner Forum. Near the end there's a link to a BMX Basics article about how to bunnyhop. Even though you already know how, reading the very thorough article will probably provide insight into what's holding you back.
Pull up, once you're almost vertical throw your bike forward. It helps to stand to one side of your bike while standing still, then with your arms pulling up and pushing forward to get the motion of the arms. It's all in the arms. For jumping remember to bunnyhop and pick the bike up to you so you're tucked, it helps you get higher.
If you do ride a lot and you're still not progressing, watch some videos and maybe do some weight training. As people have said, it's all about the arms, (and a bit about spring frim your legs), but mostly forearm, wrist and shoulder strength, or more acurately, muscle 'snap' - the ability to apply power suddenly over a small movement, especially in the wrists. The only way to develop this is practice.
Also try to practice jumping in place without a bike to perfect being a human spring... Strengthening shoulder muscles for the pull up part of the bunny hop through stuff like bouldering has helped me. The more agile you are off the bike, the more agile you will be on it I've found...