Flaking-off of paint film is due to improper application of primer coat over putty, and not being completely covered. It can be due to application of paint on insufficiently dry surfaces. It can also be due to shrinkage or expansion of a surface causing the paint film to move.
Ensure that there are no gaps in covering putty with primer coat. Also ensure that the surface is dry and clean.
Remove all loose paint by scraping and sanding and recoat.