I use a USB camera on my work laptop and the camera sometimes doesn't show up in my video conferencing app. I verified that the camera is showing in the usb list with lsusb (a script I got from https://github.com/jlhonora/lsusb which is great btw).
dtruss showed that the VDCAssistant was being accessed by my apps, so I tried killing that and boom the camera shows up.
So now I aliased camkill='sudo pkill VDCAssistant'