That is not normal behavior, was the belt replaced with an actual FT reel belt or a PF rubber? Do you mean the balls do not come out of the reel or the catapult arm cant launch them every time?

If you just did install a genuine belt and the cam and they do not come out of the reel, then it is most likely an opto issue, clean and reflow.

If it is the catapult arm not launching correctly, then it could be the cam binding, the catapult arm binding up inside the coil or the coil could be weak. Keep us posted

It does sound like it is just the opto's for the reel. The genuine belts are clear, you should be good. When they get old they slip and can create the same issue you are experiencing, if you use PF rubbers, this generally happens much quicker then a genuine reel belt.

It is unlikely it is the LM339, it could be, but it is doubtful. Start with the Optos for the reel. Take a q-tip and a tiny bit of rubbing alcohol, you may have to pull some of the cotton off first so it will fit in the tiny area in front of the opto or ideally, remove the optos so you can clean them, and swipe them a couple two three times.

While you are there you may as well flow the solder joints on the opto boards, just heat them up just enough with your soldering iron to flow a tiny bit more solder to wire pad. It can be tricky, the wires tend to move when the pad flows, so you may want another person there to hold the wires in place while you apply the solder or use another method of keeping them in place.

I would think their is a video somewhere on you tube showing the process, but it is really simple, just don't apply too much heat to the solder pads for too long, you dont want to damage them with the heat, you just want the solder to flow for a second and create the strong joint again.

Try it out and let us know, it should be a simple problem to address, with troubleshooting anything, don't read too far into it, just take one step at a time from simplest to hardest and you will fix it.