upvc door lock?

i have a upvc door the lock is giving me grief it is still locking and unlocking but is different in the following way...when the key is inserted and the door unlocked you normally rotate the key back to the vertical and remove the key from the cylinder, but on my door the key when rotated back to the vertical is starting to relock the door its as if it isnt clearing when locked or unlocked, you feel as if when you are taking the key out of the door its "almost" unlocked (the key is about to turn the lock) sorry if this sounds a bit muddled but does anyone know how this can be remedied thanks for your time