There have been threads on similar issues before.
Issue #1 - A BIG issue is if the webcams did not have unique serial numbers. They are supposed to, but as folks connecting multiple of same webcam to same computer is so rare, many vendors break USB rules and simply don't have unique device IDs. So, first place is to check at Operating System level if devices appear with unique IDs
Then does OS level tool, like Windows Recorder, see all webcams?
Make sure hub isn't your bottleneck, or your USB Root Hub on PC (depending upon port on PC connected to). Don't be surised if you need to break up cameras across multiple USB ports on PC... and even then, only if high-end performance PC to handle that many simultaneous video inputs. One way to test if bandwidth is issue is to config cameras to use a low resolution (like 480p30). If that works, but not when you increase resolution, that will help narrow down your issue
