Some times my stacks fall off when I throw, is this a problem
Well, it’s not a problem if you don’t mind your stacks falling off. 
Seriously though, is it just the plastic stack or is the bearing inside the stack coming off as well?
It is just the stacks falling off.
That’s odd. They normally fit pretty snugly. Did you look to see if they are cracked?
Do you have orings installed?
A few simple fixes
A drop of white out inside the stack will tighten the fit. Crazy glue too but make sure it is dry before you fit it to the bearing. Scoring the inside with something sharp even a push pin will make the fit snug too.
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