Hi Graham,
Did you maybe (accidentally) enable Freeze (and tick the Style box in the Freeze settings) so that the Style doesn't change? (When Freeze is on, an "F" indicator appears in the top right corner of the Home display.)
If it is not due to the activated Freeze function, either the Style itself or the folder/subfolder in which the Style is located was renamed or moved, or when the original Registration was memorized, the Style box was not ticked.
Please also note: If you sometimes have several USB sticks connected at the same time (e.g. using a USB Hub), this could be the reason for the problem (namely, when the USB stick containing the Styles is not recognized as "USB 1 " (internal drive letter "I:"), but as "USB 2" (internal drive letter "J:"), for example). If this stick is later used stand alone (as "USB 1), the Registrations would expect the linked files on "USB 2" and the Styles would not be found. So I strongly recommend not working with multiple USB sticks at the same time when working with Registrations.
These are the only conceivable reasons for the problem described.
P.S.
It would not help to save the Registrations in the folder in which the linked Style is located. The full paths to the linked files are always saved in the Registration Bank file (.rgt). So if this path changes by renaming or moving an involved folder or the linked file, the Registration would still not work anymore.
Best regards,
Chris