That used to happen to me in rift battles only, and I can say that by the hurry to finish the battle I didn't wait for the opponent to enter to the stage so that makes sense, but the last week I waited for the opponent to enter and I did the BB3 but it failed again, it seems that having the opponent cornered raise the chance to fail.
That last week I fought a Squigly in Prize Fights and they were in the middle of the stage, she was on stage fighting, she wasn't entered by defeat or tag-in and that issue happened again, the grab bag failed and Cerebella just did the opening of the BB3. Hope you can solve that problem.