I have a pretty tight ball sack, never wanted to stretch it and been using my BM for a few days now without any issue .
Unlike when I tried "conventional" vac pumps years ago. The ball sack sucking in tube issue put me off vac pumps and am surprised you have this problem with the BM.
Are you warming up enough and getting all ball junk out of the way before pumping down ?
Or maybe taking things a bit to quickly. I've found I do my first pump fairly gently to get the thing growing, off & massage, on & pump a bit harder, off & massage, on & max pump all at around 5 min intervals.
I am deliberately taking things slowly but really happy so far.
Must admit you need to pull the BM fairly hard to pump and sometimes I realise I've got a bit of scrotum skin folded, just release & sort it out before going on
Good luck.