Screw Holes Stripping
 

I see from taking these things apart and putting them together alot that some of the screw holes are starting to strip and the screws aren't going in all the way.

if you can try to get one of those cans of air and spray out the hole to get any plastic dust out and then take a small paint brush and some plastic ca glue ( not super glue its not for plastic ) and just "paint" the insides of the screw holes with it and let it sit for about 2 hours then you should be able to get some grip back with your screws.