You have a lot of different things going on.
When you removed the wallpaper did you steam them? Did you size the wall before doing any other work? Sizing closes the pores of the wall to accept a new finish. Next, did you prime the new drywall mud after sanding?
What type of paint did you use? Latex or oil base?
Different animals completely. If you used latex over oil or vise-versa ther are different ways of doing this process. Your paint store (?) may be able to help.
It's all in the prep-work though.
Your not getting the adehsion you need...just my thoughts.