Why does my iPod shuffle occasionally fail to sync play counts with iTunes?
I didn't think of posting this question until today but after a few weeks of experimentation, I've determined that, for some strange reason, iPod shuffle will only update play metadata when, before my next sync, I either


finish listening to a playlist, or
switch from one playlist to another.


If I don't finish listening to a playlist (which can be the case with very large playlists), the metadata will not be updated on iTunes.

Powering iPod shuffle off and on between listens does not appear to make a difference, but I'll report back with any new findings. For what it's worth, in normal use, these are the steps I take when preparing to sync:


Power off iPod shuffle by sliding the three-way switch to OFF.
Disconnect the earphones.
Connect iPod shuffle to the USB cable.
Sync.


And of course I always eject it from iTunes before disconnecting it.