I had the same problem a few months back. The excuse I was given was that they were unable to change tickets (I was on a domestic flexible fare and changing to the next available flight, which was still greater than the requisite 60min). Prior to this they had always handled anything I requested, with the only thing they couldn't do being to take bags once the 60min cutoff had passed.

It seems par for the course now, though. Hit or miss service backed up by random assertions of policy.