Quoted from vid1900:RAISING INSERTS:
Sometimes you find that inserts have sunk below the playfield surface. Not cupped, but actually fallen in.
Cupped inserts can simply be filled with clear coat, but if you can see the wood beneath the playfield around the edges, you need to raise it up.
Heat the insert from above and below with a hair dryer.
Keep one hand on the face of the playfield to control the insert, as you push the insert from behind with the largest socket from a wrench set that will fit in the hole.
You may have to reheat while you work.
If the insert is really loose, and you can easily remove it, do so (rough up the side edges with coarse sandpaper, prime with plastic primer and re-glue with epoxy).
If the insert is really tight, then just level it with the face of the playfield and re-glue the entire insert perimeter from the back of the playfield with epoxy.
Don't spare the epoxy, better to have a heavy coat that no ones sees from the back, than to have the insert move around in the future.