Call Steve at Pinball Resource. He has all things Gottlieb.
Replace the coil (it should come with a sleeve) and the plunger too. If the plunger is gummed up or the sleeve is worn out, this could definitely cause the flipper to hang up. It's a simple first project and not very hard to replace. Just make sure when you re-install the flipper that you keep it the proper height OFF the playfield so it doesn't rub.
http://www.pbresource.com