on vpn: quoting myself from last I looked at this (probably worth to read the thread):
If you setup a VPN, android treats it as “metered” by default. While your wifi is treated unmetered, the vpn could use the parallel mobile data connection when it is unavailable and slurp up your traffic allowance.
But on plain wifi you should see syncs after 15 minutes if there’s no failure (there are backoff intervals).
What can cause it beyond underlying connectivity? a scanning hiccup I guess when comparing the remote (“cloud”) to your file state on the machine. No idea how it happens, I’d need the device in my hands and poke at the state it holds onto.
If you’re out of patience and not in a position to go bug hunting, as you write yourself - best reach for another webdav sync client.