You have to make sure both reports have exactly the same data source / login / password AND that both reports are exactly the same width (unless the subreport is smaller than the main report).
I ran into that problem. My subreport was wider than the main report. When I fixed that, I was able to see the subreport just fine.
Catadmin - MCDBA, MCSA
"If a person is Microsoft Certified, does that mean that Microsoft pays the bills for the funny white jackets that tie in the back???