LrC doe ot sync pics in a particular folder

I am running latest release of LrC on m Apple silicon Mac. Recently I have started creating collections to sync to Lr on my iPhone & iPad. It generally has worked well, except I have one folder/collection that doesn’t sync its contents. I have tried various suggestions ( stop/start sync, delete/recreate the collection, rebuild the sync database } but nothing seems to work for this collection, Any suggestions would be very much appreciated 

Marty