Bob,
I haven't experienced significant problems, sometimes some of the mechanisms are stiff as they have been in stock in China and then Australia before being eventually shipped and then installed. could be a couple of years before they are actually used. Most of the stiff ones that I've had have eased up after a few twists backwards and forwards. If that doesn't work a very quick burst of lubricating spray should work.
Regarding undersized tubes, I haven't had any undersized tubes, but I have had some tubes that won't fit onto the bushes. This has been caused on two occasions because of the clamping machine that grasps the tube when it is cut to length. I don't know the intricacies of manufacturing brass tubing but I used to design aluminium extrusion dies, and when extruded it is impossible to get a smaller tube.
However I recently had ten ultra cigar pens from penkitsuk.com and the lower tubes were "too small". Brian was round my house and we mulled over the problem and found that when we revolved the end of the tubes it wasn't quite round. Very close inspection showed that it had been held in something like a three jaw chuck, presumably when it was cut to length and the end of the tube had taken on a slightly triangular shape, be it almost invisible to the naked eye. We measured the dia of the ends of the tubes and the middle and sure enough they were different.
Axminster sell good value 51.5mm lengths of 7mm tubes, part number, 800399,
