What causes Gelcoat to shrink

I guess I don’t have permission yet to post anywhere else yet.

My last problem was helped I thank the members here for that. Seemed I wasn’t letting the gelcoat cure long enough before laminating.

My new problem that has never happen to before. My gelcoat is shrinking in my mold and it doesn’t do it to all the molds with the same batch of gelcoat used. You see the 2nd pic both molds the same gelcoat but one shrinked the other didn’t,

Any ideas what causes this…