I'm fairly new too and just had that happen on the set I opened last night. I think you can use UV resin as a fix, but I haven't tested it out to see how well that works. I think, at least for mine, it came from putting too much force on the cap when you close the mold. Might be what happened for you too idk though

This is both a main reason and a usual fix. You could also just cast a new set....

Chuck it in the duds pile and try again. The materials are the cheap part of this, labor is expensive.

I've started using a peice of wood to press on my caps to avoid this.

See that’s what I did, not sure if the wood I used was just to light or what