Very common with these trucks. My white 96 C2500 is flaking in a few spots, as is the neighbors white K1500 across the street. I think remember one of our paint gurus on here mentioning that the primer was at fault and the paint made only a mechanical bond with it and as the paint cured (which it continues to do over time, apparently) the mechanical bond gave. I'm paraphrasing and might have got it wrong. I know very little about body work compared to the guys on here.
Point is that you need to take it down to the metal as said above.