Neil I had a similar problem with my banjo and it turned out to be lack of lubrication between the rod inside the banjo and the metal loop that allows you to move the banjo forward and back. It is a bit hard to describe but if you flip it over you will see a ring of metal that goes around a rod and slides back and forth on that rod. Lubricating that rod so the ring slides easier resolved that issue for me.

Hope that helps,

Neal