cracks in fiberglass

Hi all,was wondering if anybody has had this happen.When building tear I used 1/2 exterior ply,counter sunk all screws ect,then used 4 coats of epoxy fiberglass sanding smooth between each coat,then used automotive epoxy & surface primer and automotive urathane paint,when done there was not 1 flaw in paint job,now several months later I can see small cracks in the paint job following what looks to be the grain of the ply.and in some of the places the counter sunk screws are sorta showing up.Id like to know why it did this??????? Thanks 
