Evan Gatehouse wrote:

Yes - I think you got lucky the other times. Gelcoat is NOT supposed to
cure when applied over epoxy (i.e. the conventional wisdom)!

My experience has been that the "conventional wisdom" is simply wrong
and epoxy manufacturers agree. Once epoxy cures completely, it's inert,
so it shouldn't matter what you put over it unless it specifically
reacts with the epoxy chemically. I've used this particular combination
of epoxy and gelcoat successfully many times (literally from the same
cans) and have used other epoxies with this gelcoat, so I know from
experience that it works quite well.

Something has gone wrong this time, but I can't figure out what it is.
One suggestion I've been given is contamination in the epoxy or gelcoat.
That's possible, but I'm not sure what could have contaminated it.