I’m pretty sure that you’ve already checked, but the obvious things sometimes fly under the radar and go unnoticed: is the phone in file transfer mode in the first place? Other one (which has bitten me) is if you’re using an usb-hub, try direct connection and/or different ports on the host computer.
Personally I’ve spent far too long to try and hunt down something obscure while the fix was really simple as some default option changed with updates or whatever. And in general I’ve forgotten to check the simple things first way too many times and that has caused wasted hours way more than I want to count or admit.