Since most inserts have slightly tapered walls, they will naturally want to self-eject over a period of time.

Many glues don't penetrate (hehehe i said penetrate) the plastic enough to hold it in place...in fact the insert breaks free from the glue and slips out again.

A good practice is to roughen the outer edges with coarse sandpaper, essentially creating fine scratches on the outer wall that the glue will then get trapped into, and create a far more secure grip into the playfield hole.
In extreme cases, you can poke around the edges of the insert with a soldering iron, creating small craters as such, which also creates a glue trap and greater friction against the playfield wood surface.
Yes, they'll be harder to get back in, but they'll NEVER come out  ;-)

The scratching & soldering iron methods don't show in the insert bulb-area, so you afford to be brutal with those edges.

MM