It could be one of several problems.
The reels could be out of alignment so the payout fingers are not going in the payout disks.
The slides might not be resetting at the beginning of each pull.
You might have a coin that fell into the mechanism which is preventing the clock from cycling properly.
The spring connected to the clock might not be strong enough to pull the clock all the way to the end of the cycle.
If you are mechanically inclined look at the mechanism. You should be able to figure it out.
Also, you might want to go to my web page (
http://www.IBuyOldSlots.com) ad look at my videos on how to unjam a slot machine.
Dave