I had some hobbypoxy do that before as well. Not quite sure as to why. Fortunately I only use, hobbypoxy is what I call hobby grade/ brand epoxies, for filler's. Not for anything structural or lasting. Go to System Three's T-88 epoxy and you won't ever have to worry about it. Plus it's ten time's better. And, while your at it,  try their GelMajik and you probably will not use regular epoxy again.