+xABS is virtually impossible to bond to, and also have quite a high CTE. When you put them in the oven, the parts are expanding with the heat. Carbon has a very low CTE, so the difference between the two materials, coupled with the fact the epoxy has a very poor bond with the ABS, results in the carbon delaminating from the plastic.
ABS requires very thorough keying prior to applying the base resin, ideally with 40 or 60 grit sandpaper and even scoring it with a blade. Even then, it's not likely to stay in place if it is subject to thermal cycling, those differences in CTE just cause too much stress.
Thanks, different CTE makes sense. But what might be cause of epoxy sinking through 3 coats which were already cured in the oven and been sitting for 9 days.