Hi Ray,
This happens alot to me and as Martin says easing off the tension on the retaining screw will do it. I would suggest though to file down both sides of the cam as easing of the screw can cause the barrell to go loose after a while.
A couple of points though to be aware of....
Sometimes the height of the cam on the cylinder can catch in the case as it travels to the top, then you have to file down a bit of the cam, this happens alot on aluminium doors.
If you do ease of a barrell on some Anglian doors the screw can block the travel of the gearing.
Some aluminium doors require like for like barrells to be changed because of the unique shape of the cam, you see them most on the oval cylinders.
I recently changed a barrell on a upvc door lock and it would not work correctly, after opening the case to see why the mechanism would not work, I noticed the cam only connected to about 2mm of the mechanism and if the barrell would move a touch the lock would not work, I fitted a new barrell and suggested he change the upvc strip, he didnt and rang me again because he could not open the door, I changed it again and he kept the same upvc strip in

The upvc lock was a avocet and the quality was very poor.
Don