Using a PC with Windows 8.1
I''v actually spent HOURS on the phone with Apple over this problem. Other hours at the Genius bar and have emailed diagnositce to Apple (yesterday was most recent)!
Finally, went back today, played around and came up with something that worked, at least for me.
I'm synching from Itunes (latest version) to Iphone, mostly audiobooks Albums will synch to Iphone but some tracks will be grayed out.
What wotked for me was to:
1 right click on the 1st track with a grayed-out circle
2.. on the menu that comes up click on "Uncheck Selection" (or "Check Selection", if that appears)
3. Right click on the track again.
4. on the menu that comes up click on "Check Selection" (or "UnCheck Selection", if that appears)
The tracks should now begin synching. If they don't, go to the next unsynched track and repeat the process.
Now, somtimetime all of the unsynched tracks will synch and Duh!, some tmes only some will synch.
Keep repeating the process until all are synched.
Good luck.
Ed