Canon brand? I ask because 3rd party versions have worse tolerances and have been known to be super tight or loose. I have not had this issue yet I have had a sticky button on an EF 1.4 TC. First thing is to take it off the body and put caps on. You wouldn't want to try anything with it mounted to the camera. For me the main thing is to be certain to not damage the lens (the adapter could be sacrificed if anything had to be imho) Try pushing up on the button as far as it will go and then a bit more while gently turning the lens in the direction for release. If it doesn't unlatch, try the opposite direction. There is always a bit of wiggle room with these matings and you shouldn't be too worried about putting a bit of muscle into it, not too much mind you. Once you get it off, see if you can toggle the button back and forth, check for obvious things like broken parts and bent parts. If it's an ultra tight mating between lens and adapter, it might be the mount. If it's Canon brand, they will be easy to work with (In the US) Good luck.
