Mitel 5324 Headset key not working

[KaptenBlack]

I have a 5324 IP phone connected to a 3300. I have tried programming a headset key on the phone and it does not work. The Key saves but does nothing when pressed. I then programmed the same Key on a different 5324 and it worked as I expected. So the issue seems to be localized to the handset. I assume there is something else that is stopping it from working. Does any one have any suggestions of what to look for?

[sarond]

First thing I would test is making the phone that is not working the same COS as the working phone and try again.