Mac Studio files do not sync via iCloud Drive

I bought a Mac Studio at the end of the summer to replace my 27-inch iMac. I also have an Apple-silicon MacBook Pro, an iPad Pro, and an iPhone 13 Pro. I share my Desktop and Documents folders among these devices via iCloud Drive. Files sync swiftly among the other three devices, but not with the Mac Studio. Files created on the Mac Studio might not be synced for days (at least), and files created on other devices might not sync to the Mac Studio for days (if ever).

I can't figure out why this isn't working. I called Apple Support and spoke with someone, who said he'd send me a case number and some information, but I never received anything. Can anybody offer any suggestions?