What went wrong?

Hi y'all, I'm working through the goblins and the basing has been coming off a few of the minis. I scrubbed them clean before the basing, used a reasonably thick coat of school PVA, sealed the whole thing once dried with a second lighter goat and then primed.

I've glued them back on and it seems to hold for now but would quite like to understand what happened so as to avoid the same issue for the next batch.

Any ideas as to why that happened? Thx!