Sounds like you're experiencing shrinkage of either the epoxy or polyester resin (not certain why you're using both in combination?)

This would happen more readily if the parts are exposed to heat above ambient temperatures after the initial cure, especially if exposed to strong sunlight. One way around this would be to post-cure at an elevated temperature following the initial coating stage, then do the final sanding to take away the uneven surface

Skinned parts generally are not so resilient to shrinkage, given the additional thickness of the resin coating, especially if exposed to natural light and heat etc.

Again, it's most likely slight shrinkage of the resin following initial cure

This will be more apparent if exposed to heat and a post-cure at elevated temperature is ideal. The parts will show signs of shrinkage after a post-cure, but doing the final flatting after that should give better long-term cosmetic results