Change Polling sometimes doesn't run when there are pending changes

Occasionally Change Polling won't start, even though there are pending sync changes showing.  Running a Baseline Sync clears the issue and subsequent changes make Change Polling work normally, so the workaround is to always have periodic Baseline Syncs scheduled in the solution.

Sadly, I have no idea what causes this or how to replicate it so it is likely to be quite difficult to track down.