The controller can't tell of you've entered the wrong format for the flight number in your FMC. On more than one occasion over Oz I've heard quite a VHF exchange going on to workout why the CPDLC won't connect. Only after quite a few calls did they workout it was the flight number incorrectly entered in the FMC causing the failure to logon.
Problem fixed
The logon is initiated by the Aircraft not the controller, hence the secure connection never works if the data is wrong from the Aircraft and the controller cannot see what the problem is.