Hi Katrin
I don't know how to fix it, but looking into it a long time ago I figured out that Koha uses only 245$a for sorting on Title and filed a bug report to improve this behavior:
*Bug 15161* <https://bugs.koha-community.org/bugzilla3/show_bug.cgi?id=15161> - Sorting by Title does not include volume/part information from $p and $n
What you might see happening is that Koha sorts by biblionumber, when the first sorting criteria is the same for multiple records.
Thanks for the information! Yes, this seems to be the problem. Even if currently there is no solution it is good to know the reason. And what I often tend to forget: Only because MARC has some highly or even over-sophisticated fields, it doesn't mean Koha is using all their features for creating, showing, sorting them in every way I could imagine. Best wishes: Michael -- Geschäftsführer · Diplombibliothekar BBS, Informatiker eidg. Fachausweis Admin Kuhn GmbH · Pappelstrasse 20 · 4123 Allschwil · Schweiz T 0041 (0)61 261 55 61 · E mik@adminkuhn.ch · W www.adminkuhn.ch