This is a problem for me sometimes, too. Exactly what is wrong?
I know my friend was having an issue where the WiFi Music Sync wasn't running on his computer, even though the Desktop Software had the option for it to be running checked off. What I did was un-check that option, and hit the confirm button. Then, open it back up and and check the option and again and it's running like it should be. It seems that (for me and my MacBook anyway), once the phone recognizes Desktop Software and becomes mounted to the computer, it turns off the Music Sync from the computer.
Maybe that can help.