I had a similar issue with my rig. The first thing I'd recommend is don't do anything crazy like reinstalling windows. The solution is usually the simplest. In my case I solved it by:

1. If you're using two PSU, make sure your gpus share the same psu, don't mix them (pus powering a riser different than the gpu's psu).
2. The gpu that was giving me problems to yours, I connected it by itself on the first pcie slot and installed the drivers. Then I connected the other gpu on the second pcie and installed the drivers. After that, I plugged the rest and windows recognized all.
3. Try one gpu at a time, then keep adding. I'd start with the problematic one.

Make sure to get the DDU app to completelly uninstall nvidia drivers if everything fails, and try again.