Graham,
I ran into a similar problem recently.
For whatever reason, the tape name and the archive label can not be identical and when they are, the BRU PE gets confused.
To correct this on items you’ve already archived, go to your restore tab, right click any of the tapes/tapesets and change the name by one character.
When the column refreshes, and you click on the disclosure arrow, your archive catalog should magically appear.
To prevent against this in the future, just make sure there’s one character of variance between the two.
Hope this helps.
Ian