This is easy and you can do it yourself. Model 12's are easy to work on and are robust enough that you can't do too much damage.
Ensure the gun is unloaded first, cycling the action.
1) Disassemble the M12 in normal fashion. Pull the trigger on an empty chamber, push the magazine locking pin downward and toward the right, pull it out until it stops, use the pin as a lever, rotate the magazine tube counter-clockwise (1/4 turn) until it stops. Pull the magazine tube forward.
2) Move the slide action and forend assembly forward until it stops, rotate the barrel and magazine assembly 1/4 turn counter-clockwise (front view) Remove the assembly toward the front.
Taking the forend/magazine assembly, use a wooden towel or a pencil eraser-end first and try to depress the magazine follower inward and see if it binds anywhere. You could have a broken magazine limit plug, grit, crud, gunk, etc. in there. NEXT -
Remove the magazine spring, plug, follower:
a) Take a fine screwdriver and depress the round-wire spring in its slide-slot on the underside of the takedown pin at the end of the magazine tube. By depressing the rolled-tip into its recess, you can remove the takedown pin and slide it OUT of the magazine tube.
b) Remove the two small opposed screws at the front of the magazine tube. Take out the magazine plug and the plug stop plate toward the front. CAUTION: The magazine spring will be released, so restrain it and ease it out. Remove the magazine spring, plug and follower toward the front.
Check to make sure there are no binds, bends or kinks in the spring. If there are, get a new one.
Clean out the tube of debris like you would a shotgun barrel with a bronze brush and some cloth soaked in cleaner. Check clearance and freedom of movement of the follower (which is steel) by dropping the follower back into the tube and see if it glides smoothly up and down the tube. If there is rust on the follower, you can easily clean this up with some 4-OOOO steel wool and WD40. If you get it to ride freely in the tube, you've succeeded.
Assemble in reverse over.
(Note- when reassembling, put a couple of drops of Loctite on those two opposing screws at the front of the magazine tube. They have a tendency of working themselves loose at times.)
Good luck!