I ran into this problem this week on a TWD - flipper buttons can be pulled out - poorly made apparently.
What I did to fix it was fire up my soldering iron, and slightly melt the end of the plastic to deform the edge slightly so it wouldn't pull back out. That seemed to work well.
If anybody has a source for new buttons, let me know - the ones that everybody seems to sell are slightly longer than what Stern is using now.