CC reels are probably the simplest score reels ever designed. That said, they are probably the most problematic score reels ever designed. I know that probably doesn't make logical sense, until you have problems with one and can't ever seem to get it to work.
There are some wiper fingers that travel around a bakelite disc with copper traces on them. These fingers need to have enough tension to make contact, but not too much to cause drag and bind the reels. The 100s reel activates the 1,000s reel when the 100s reel goes from 9 to 0. But not only do you need to make sure the 100s contacts are clean and making good contact with the wipers, but that the 1,000s reel itself is clean and can easily rotate. You can add or remove tension to the wiper fingers, as required, by slightly bending them and clean the copper traces with some scotch-brite pads. I add a little bit of lube to the copper traces to reduce the wiper drag which helps a lot. So check both the 100s and 1,000s reels.
I have 1 CC game and had the same issue as you, but luckily I only have a 2-player game. It took an entire afternoon of continuously adjusting tension, cleaning traces and light lubing the wiping surface as well as the reel mechanisms themselves to get them to be reliable.