Can't say I've ever had that happen with my troll, though it has happened with other atties. Cross threaded screws will do this. Screws with worn or even chipped threads will do this. It's pretty much an inevitability with how much heat and abrasion those cheap, shitty little guys have to endure.
Sometimes it can be caused by bad threading in the posts too. My Mutation X v3 has one post that routinely eats screws. They hit a certain point before they drop and spin freely. And once that happens, they're no good anymore... ...in THAT post hole. It'll secure fine in the other 3.
Not sure what to tell you if its the latter. Maybe the screws aren't quite the right pitch. That is bizarre. Sounds silly, but a temporary fix may be to rotate your screws like you would tires. That's what I keep doing with the MX v3. I have an unlimited supply of aftermarket hex grubs for that atty, though.
I've actually been wanting some higher quality screws for the troll, myself. That might solve your problem, too. Just wish I knew what the dimensions were. I have hundreds of different metric grubs and none of them fit