Maybe it wasn't enough flouride when they were baby gears but sometimes the teeth in a gear can break off. This of course will stop the clock because the gears will jam when trying to mesh together and spin. It's not all that uncommon in the wooden geared clocks because of the grain in the 150 year old gears just cracks and a tooth shears off. It happens in brass wheeled clocks too though. It could be just from a weak spot in the brass since new, or with cast gears, a flaw in the original casting, but with brass gears it is usually from forcing something. This could be pushing the minute hand that is stuck for one reason or another, or forcing the strike gears by pulling on the weight or something.
However it broke doesn't matter, I'm just showing how I repair them.
On a wooden wheel a piece of oak or maple or birch or something hard is dovetailed into a notch filed or saw cut in the gear. Even a perfect tight fit should still be glued as a safety measure. Not shown is a final picture of the tooth stained to match the gear. Forgot to take that pic.
Before and after
Pictures of before, ready with blank, and finished. Replacement tooth is at 12 o'clock.