Found the XSLT problem; it was caused by two different abbreviation forms which are marked up in the same way, which is a problem for the markup team (and passed on to them); however, my code should have been trapping for it, and that's now working. The status now is:

• 96 poems are functioning correctly WITH apparatus.
• 89 poems are functioning correctly, but don't have apparatus (either because it's not done yet, or there are no other witnesses).
• 6 poems have a problem with text appearing mingled with the line numbering.
• 1 poem has a block of text in an odd font.
• 1 excerpt (the last) appears to be missing, or has no title and so can't be accessed easily.