Garage last painted 15 years ago. As I apply new paint the old paint peels off of the wall and sticks to the roller. The walls are drywall.
I will offer a maybe as to why and then suggest a remedy. Too much moisture will always cause paint to peel from drywall.
Remedy as crazy as it may sound is to use a damp (water) roller and cause the rest of the paint to peel. After what will release has released, check in with you local paint dealer and ask for an appropriate primer. Prime and finish with at minimum 2 coats of finish.