Iphone won't backup to computer using itunes

I recently had to do a full restore of my iphone. Afterwards, I used my existing backup to restore my data. This was about a week ago. Today I tried making another backup of my iphone, but it won't. It says the phone was never backed up on this computer, which is not true.. I can also still manually locate the backup data stored on the hard drive.. and when I click to back up, nothing happens. It starts to sync for a couple seconds and then nothing. I have tried fully uninstalling/reinstalling itunes. How can I fix my phone/itunes so that my phone will backup to my computer?