Since you have the bottom cover off, the first thing you need to do is clean the magnet.
I use a piece from a dollar bill, just clip an edge off. Wet it with alcohol or contact cleaner, put it between the armature & magnet & pull it out. Between the solvent & rough texture of the paper it may be enough for it to work reliably.
If the diaphragm is oily, you should see some on the blades, but you're most likely correct.