Scrapped it out from my answer on cognos communities, maybe it’ll help someone.

Drill-through is really to tough to debug.

Prior to 8.3 I’ve used following routine:

1. Set the target report filter to optional
2. Add 2 new text item to target report, and change them to ReportExpressions of ParamDisplayValue (?param?) and ParamValue(?param?)

This will show you what is passed and, maybe, will clarify things a bit.

Since 8.3 you’ve can see passed values under the link you’re pressing — really helpfull.