I know Im late to this party but I was having trouble connecting to my XIAO and I found out what the problem is in my case.
I had bought some JBL headphones that came with a USB C cable so you could charge them. It is orange and flat, I tried to connect to the XIAO with that assuming it was like any other USB C cable.
Turns out some cables are ONLY for charging, this one has no markings to indicate that. I swapped to a round USB C cable and the COM port comes up straight away and everything is working as expected now.
YMMV