Extra balls might be binding the one that should be shot out. Not likely unless something is severely it of alignment though. Buzz would be the coil trying to fire and just being stuck. If the balls are stacked to come out at once then maybe the coil is damaged/wrong and is too weak to lift them all.

Yup. I had an issue that sounded similar to this. On the bracket under the playfield, make sure that the backstop (the flat piece with the rubber grommet) isn't bent out too far. The coils under there have limited power, and if the backstop is bent or missing the grommet, it won't have enough electromotive force to pull the plunger in. The issue with my friends game was that we could hear the coil, but nothing actually moved!

Best way to test this theory is to get into coil test with the playfield raised and move the plunger into the coil manually. If it catches and pulls in, gently bend the bracket in. Also, not having the rubber grommet would have the same effect.

Good luck!

Marc