Might be a dumb question for te gunsmiths on here, but here it is: my son has a CZ O/U that about 40% of the time fails to fire the “under” barrel, the primer is slightly dented but doesn’t fire. Anyone have a fix for this? Apparently, it is a problem with CZ’s. Any suggestions appreciated.

Change out the firing pins, return springs and hammer springs. make sure FP channels are clean and free of lube. Inspect hammers for damage and wear at contact points. Inspect sears for damage at wear points. We get a lot of used O/U's in the shop folks buy and have snapping issues. Almost always the result of someone trying to gunsmith them. We have a SKB in the shop now that had all of the above parts ground and or stoned on.

Some Miroku and the corresponding model Browning O/U shotguns also suffered from light primer strikes on the under barrels. The issue manifests itself with some brands of ammunition having hard primers.
Due to design, the under barrel firing pin in many O/U shotguns guns is hit by the hammer at an oblique handle i.e. not a straight on hit, and the under barrel pins also usually have a light spring installed to retract the pin to prevent drag on the shell head as the gun starts to open. The top pins are just floating.
All these factors including any build up of crud and lubricant come together to create light primer strike.

I have never had any issues with my own Miroku O/U shotgun but there are/were after-market firing pins available here in NZ for these Miroku/Brownings called Stickyrib and having a 20 thou extra protrusion to get over light strike issues.

The issue with your sons CZ O/U sounds very similar to that with the Mirokus and Brownings. Replacing all the springs and pins is probably not necessary at this point but a focus on a good clean out of the mechanism and replacing the bottom pin (possibly has a chipped tip after some use) should solve the problem.
Trying some different ammo may help solve the issue too.