Dont get discouraged, thats normal while learning. I was the same way, one day id get them up, the next i couldnt get the wheel of the ground. Its just a matter of practice, the big thing with wheeling is technique, a very minor body input(leaning forward subconciously) when doing a wheelie has significant effect on the bikes response. You will have to consentrait to purposely not do this, as it is instict to put weight forward without even realizing it at first when you start bringing them up. As long as youve done some research and have a good idea of what you should be doing, cover brake, CLUTCH, and focus on rolling back WITH the bike and not fighting it, all thats left is to get out there and practice, it will come. Just understand that its not the bike, its youre technique, focus on that and you have no option but to progress, be safe and good luck.