Besides the typical Dodgy insert that keeps grabbing hold of the plunger.....
I found my main 'sticky' bumpers were issues with the "ring & rods" piece:

1) Rods covered in oil/dirt and binding within the playfield wood (perhaps oil swelled the ply, choking around the rods?)
2) Rods binding against the playfield hole, due to mis-aligned mounting of bumber metal bracket
3) Ring/Rods slightly bent from ball slam or owner mis-handling them/alignment?

Also replace the saggy return spring.

I haven't seen the magnetised problem before...

I rebuilt all mine as a total overhaul (i think the only original parts are the coils and main mounting bracket).
I figured if i'm going to the effort to pull apart the bumper to replace an insert sleeve, i may as well refresh the fiber yolks, rods+ring, springs etc etc.

It's amazing how much snappier they become, almost scary.

MM.