If the plug won't lock then it really should not let you charge in either.
Look carefully around where it plugs in. I had an issue on my bike where it was not making a good contact, similar to what you are saying, and it turns out a small screw fell out of something somewhere and was laying in there next to the plug, so when I tried to plug it in, it was pushing on the screw sitting in there sideways, which was not allowing it to fully seat, the switch was not made and the error.
Make sure the plug is good, make sure everything is tight and make sure there are no obstructions around the plug holes etc etc.
Also, Ive had times where I thought i plugged it in but I did not, it was cocked or something and it failed on the initial handshake, so make sure you have it aligned just right and the female part of the plug (bike side) did not get loose or wiggle out of alignment or something and the plug is FULLY seated down FLAT and not resting on something.
Aaron