I’m having issues with stomping the brain bugs. Whenever I butt slam the brain bugs nothing happens. I can’t figure out what to do for it.
Are you in the part of the mission where the objective literally says “slam”?
You also have to shoot and melee them, IIRC.
Yes.
I slam the ground between two to get them both at once and haven’t had an issue with it, don’t know why you’re having trouble with that…
You have to wait for them to reform. If you bash them too soon they won’t reform.
Also pay attention to the type of bash. You have to shoot, melee, and butt slam them. Watcht the prompts.
I can attest that there is an occasional issue. For me, the first 2 characters I did the mission with had trouble shooting them (and it counting). Even after waiting a good minute or so in between attempts, shots directly into the mass did not have any effect other than causing the piles to crumble again. After multiple attempts at different piles and ranges, one finally counted. Then the nightmare began again, but I eventually got enough to count to progress to melee. My third character, however, had no issues whatsoever.
if havin issues my guess is to leave map and go back / save & quit then go back or find a friend / rando and join their game. if none of those work you are effed good sir 
borderlands is known for plot holes and glitchy ■■■■
If you have a skill-based bonus to slam damage, like Element, you might have trouble with that objective. I was trying with my Claptrap while he had “Cryogenic Exhaust Manifold” and it wouldn’t count. I had to remove the skill and then it registered my slams.