There are two things that may be going wrong. First, you are pressing the sync button incorrectly. Make sure you are pressing the small white button with the wifi symbol on the guitar AND the connect button near the CD port on the Xbox at the same time. Hold them both down for a few seconds and the sync process should start.
If that did not work, your sync component could be broken and will need to be replaced.
Check out the troubleshooting page [[Mad Catz Rock Band 4 Wireless Fender Troubleshooting|here]] for any additional questions.