I just played a song for the first time and it went from 0 plays to 4. Sync failed a few times, then I reset the phone and tried again, successfully. bluenose56 gave me the clue to resolve the problem for myself, and perhaps others. He wrote: "A couple of times iTunes said it had to restart the synch so the play count was incremented again!"
Indeed. My iPhone sync has been totally unreliable since iOS 8. I don't even try wifi anymore, and even USB fails routinely. The only way I know to ensure a correct sync the first time is to relaunch iTunes, restart the iPhone, and plug the iPhone into the Mac.
So, I had that song that went from 0 to 4 after attempting to sync 4 times until a complete sync was made. Without unplugging the phone I played another unplayed song, and since I hadn't unplugged the iPhone the sync performed correctly. 1 play count was recorded.
Then I unplugged the phone, played another unplayed song, and plugged it back in. iTunes didn't mind that time, and synced on the first try. 1 play count.
Repeat: unplug phone, play an unplayed song, plug it back in sync. This time the sync failed. I relaunch iTunes and restart the iPhone in order to ensure a good sync, and try again. This time the sync works. Expected play count from that one play: 2. Result: 2.
In my particular situation with this issue, the kludge is to remember to restart the iPhone and relaunch iTunes before I sync.
By the way, this is screwing up your scrobbling as well, if you haven't noticed.
z