That contact at 12 o'clock has enough rust around it that it may contacting the ground around the insulating disk. That would make it heat up, and also divert some of the voltage from the coil so it won't move the diaphragm. You might try a soak in Evaporust to get rid of the rust. Evaporust shouldn't hurt any of the other parts, and it can't hurt to try. It's' also possible that the armature is stuck inside the coil. Maybe a little percussive maintenance will free that up, as well as the rust removal with Evaporust.
