This problem is likely caused by a Unimount Key Pin that has shifted and is no longer centered. Typically you will see this as a Unimount Key that does not sit straight in the Unimount, but is instead at an angle.
Firstly, do not force the Key if it feels stuck. It should be able to be installed and removed with relatively low force, but will remain solidly in place during use. To fix a Key not locking out, rotate the Unimount Key so that the TEMPO text is vertical, then gently pull outwards to remove the Key from the Unimount. Inspect the Pin position. If it is not centered, simply push the pin back into a centered position with a thumb or finger and try again.