DOCBOOK-APPS: Checking citations not working properly

[Colin Paul Adams <colin at colina dot demon dot co dot uk>]

Using DSSSL style-sheets 1.56, with (for both print and html style-sheets):

(define biblio-citation-check #t)

I keep getting several messages of the form:

openjade:/usr/lib/sgml/docbook/print/dbinline.dsl:132:2:E: Cannot find citation: A History of Chess

I cannot identify any correlation between those entries that pass the
check, and those that fail.
All bibliography entries are formatted correctly, whether or not I
turn on the checking.

Any idea what may be going wrong? How should I try debugging the problem?
-- 
Colin Paul Adams
Preston Lancashire