Getting your back wheel up is all about technique and strength in a particular bit of your leg, which unless you've been practising bunnyhops/brakeless endos for a while or working out in a gym, probably isn't that well developed.
Speaking of which, you're more likely to get the hang of pulling up the back wheel if you practise just doing endos for now. Once you get a feel for pulling the back wheel up with your legs, then go back to bunny hops, which are the same thing but with your front wheel already in the air.
Or read the link in #5.