The screw at the servo end or the bellcrank end? I noticed the one at the bellcrank end trying to work loose on me despite the nylock insert-nut, so I changed out the button-head screw for a caphead screw 1/8" longer than the original, so that I could get it tight enough without stripping out the head, and having enough thread to go all the way through the nylock so that I knew it was gripping.
I also put an aluminum #4 washer between the ball-link and the arm on both ends so that the linkage lines up a bit flatter from servo to bellcrank, and you get a bit of squash-grip between the washer and link ball when you tighten things.
https://cimg0.ibsrv.net/gimg/www.rct...5f8b7302e1.jpg
Thanks for the idea, that is exactly what happened to me