I had a similar problem a couple of years ago at an EA CCS charger when renting a Bolt. When the charge stopped (at 100%) the connector refused to unlock. Two of us tried everything we could think of for the next 20 minutes and no dice, but then the guy helping me gave it a last, angry yank not expecting anything to happen. It had unlocked itself at some point shortly before, so he almost fell on his butt. No idea why it happened then, and I haven't experienced another incident since in the 3 different BEVs I've subsequently rented, but I've never charged at that same site since then. Tried to once and the chargers there were apparently down, as I couldn't start a charge. Fortunately I didn't need one, so drove to another EA site closer to home and got one with no problem.