What i did for the magnetized pop bumper assemblies on my Black Hole:

Layer a couple small pieces of scotch tape on the surface of the coil stop where the plunger makes contact. Made a world of difference in the performance on my game-no more sticking pops. There's probably better/cleaner solutions, but it's done great for my BH so far. I even went ahead and did it on the pops i wasn't really having trouble with and i swear they also work a little snappier in result.

Hopefully that helps if you're still dealing with magnetized plunger/stop, which may be possible...

Also, as zacaj asked earlier, does the coil continue to hum after it's activated and stuck? Or is it quiet, but will only release if you manually pull it up?

Ahhh, forget about magnetized stops/plungers for now--i overlooked part of the thread that pretty much addressed that.

Well if it's continuing to hum, it may not be opening the end of stroke switch as zacaj was mentioning earlier. With the game off and the playfield opened up, move the pop bumper rod and ring up and down manually, you should see a switch on the pop bumper mech open as the rod and ring is pulling into the coil and closing as it springs back to its resting state. Sometimes a leaf from that switch can get on the wrong side of the pop bumper yoke that activates it.

If the switches are adjusted properly, next thing to check would be whichever relay activates the pop bumpers. There may be a switch on that relay that needs adjustment. Not sure which relay that would be, but you should be able to find it on the schematic.