HTC have recognized that iTunes is the largest online music store and management system, and have provided the useful facility to sync selected playlists or all songs from iTunes to your HTC phone via HTC Sync. However, if you, like me, have the same problem whereby it only copies a few songs, follow these simple instructions:
  1. Step 1: Right click the HTC Sync icon in the system tray, and click Exit. 
  2. Step 2: Press the Windows Key + R. In the box, type %appdata%  and press enter. 
  3. Step 3: Highlight the two references to HTC in the window which appears, and press Shift + Delete. 
  4. Step 4: Re-open HTC Sync, select what you require to be synchronized and click Sync Now.
  By resetting the software in this way, any problems you were previously having should now be solved.   Disclaimer: Although the instructions above should not cause you to lose data, I cannot accept responsibility for any data loss which you may encounter.