Quoted from wizardblom:
Clean the player unit in the backbox.
Most of the time they are dirty giving these problems.
It is recommended to do so on all stepper units, rules out a lot of problems in the future.
I'd go even further. I just finished an Aztec which was my first EM and had been sitting in a barn for 20+ years.
My rule of thumb is to not care about playing the game (if it's been sitting unused) until I've cleaned every switch, score reel, stepper and anything that has a coil and plunger. Manually actuate all those components too. Stepper units and score reels should all move freely and once you've disassembled one or two the rest are pretty much the same. Take pictures if you think you might forget. Although I did keep the score motor cogs together while cleaning.
Obviously, this is hours and hours of work but when I finally turned that "clean" game on everything worked except for one misadjusted switch on the score motor (which was probably my fault for not seeing how it's supposed to move correctly). A lot of stacks can play tricks on the eyes. Waxing the playfield is one of the last things I do and not until the rest of the game is as clean as I can get it.