By any chance do you leave the key in the otherside of the cylinder when locking? this will cause the cam inside the cylinder to sometimes turn 180 deg and then deem the cylinder unusable and the key stuck inside, so you cant remove the key when in the unlocked position
www.upvcspares4repairs.co.uk have reasonably priced anti snap- anti bump eurocylinders if you do require a new cylinder
Anthony
June 2010