My DB Write Where Clause looks like this:

ORDNUMBER=%ORDNUMBER AND DTCSEQ=%DTCSEQ AND MEMBERID=%MEMBERID

but for records where %ORDNUMBER is blank (which is a valid value) the where clause does not locate a record.

I've tried a number of different combinations in the where clause and how %ORDNUMBER is populated with no change to this behaviour.

Any advice?

I don't have access to SQL Trace otherwise I'd run one.