Since the consensus of opinion here is that it wasn't my fault, I contacted Turner's Retreat who, without demur, said that they would send me a replacement for the faulty blank. I hope this one turns without any problems! They said that they had had difficulties with this supplier and have now ceased stocking these Chiyogami-Yuzen blanks. Probably a wise decision.
I have bought a lot of stuff from Turner's Retreat in the year or so that I have been turning pens and have found their customer service to be excellent. Ordering from them is a pleasure, although I am not sure that my bank manager would agree!