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Sometimes it is just a matter of getting it spinning again after a long rest. With the cover off and the horn button pushed take your finger and give the armature a push to get it moving. Often that is all it takes. Also if you do get it to spin, place a small piece of fine grade sand paper against the area the brushes contact the armature to clean it up. Doesn't take much.
Your adjustment screw can be very sensitive. Usually only takes a few clicks one way or another to adjust the horn.
Don't be too aggressive in tightening down the horn cover screw. Too tight can affect how the horn works.