A sticking solenoid on a pop bumper are usually caused by one of the
1. Switch gap too close. Regap and you're done.
2. Dirty switch cup. Undo the screws that hold the switch stack in place
and clean out the dirt. This cup needs no lubricant. Sometimes some
foreign matter sticks inside the cup and interferes with action. I once
found some solder splash inside one of these cups!
3. Misaligned switch cup. Check that the bumper skirt post is at the
center of the cup when at rest. If not, then you need to loosen the switch
stack, either where it attaches to the playfield, or just the bracket itself
and align the cup. If this is out of center only slightly it will make the
switch more sensitive on one side. If this is too far out of center then
the bumper skirt post will catch on the outside edge of the cup and get
4. A bad coil stop. I've only found this to be a problem on Gottlieb pop
bumpers, but it could happen anywhere. Just replace the stop and see if the
problem goes away. If you don't have a spare coil stop, just swap the stop
with another pop bumper and see if the problem moves.
5. Lubrication. This is the worst reason. These solenoids should never be
lubricated. They just need to be clean.