Problem Solved!!!!

The missing subreport: I'd been filtering the entries to the Main report (perhaps I should have mentioned that) while I worked out the bugs. I used a criteria for the location in the query that the Main report references. There were no data entries for the Level subreport for those Locations. I stuck some in and now the Level subreport appears. The Level subreport appears only once per Location (as it should). ...

I have a Main report in my database with two Subreports ("Level" and "Quality"). Each (sub)report is based on a different query, but all queries include the same field ("Location") from the same table and it appears on each (sub)report for debugging purposes. This field is the Master/Child link.

I have run each subreport independently (both have controls only in their Details section), and they function...