If a particular collection is synced, it would display that lightning bolt symbol on left of its name.
If that symbol is not there, right click/command click on the collection name and choose "Sync with Lightroom Mobile" and make sure Syncing is turned on.
Click on the dropdown menu next to Identity Plate and check if the syncing is turned on. It should not be paused.
Thank you, Chetna. I thought I had done that yesterday. Oh well . . .
Glad to help