I've discovered a clue as to why the ftm to ancestry sync fails. Even if I make substantive changes (adding or deleting people, facts, media etc.) instead of the normal 'Sync Change Log' with the numbers of things affected I get a different 'Sync Change Log' with the following message:
"These changes are need to bring the trees into sync.
The changes in this sync update are related to general tree structure and are not specific to data for people, media, or source."
It appears as if the software misinterprets substantive changes.