A few quick hypotheses off the top of my head:

1. A filter in the report tablix might be removing the results

2. A visibility expression in the group/tablix might be hiding the rows

3. The data source in SSRS might be pointed to a different DB

4. The default user in the data source might not have permission to run the query

5. If you're looking at the report in SSRS using Chrome, it often won't show any results (I use IE for SSRS)

6. There might be something wonky in how SSRS is passing the parameters to the query - make sure the data types match.

7. If any of the parameters are populated by datasets, the queries for those datasets might be incomplete. Let's say we have a parameter @store, and I have a query for that parameter which lets me choose store 1, 2, or 3. If there is also a store 4 and I enter it, the report will bring up no results because it doesn't think that's a valid value (this might just be a 2005 issue).

8. In Visual Studio 2005, you sometimes have to refresh the datasets to get new data to show in the report.

I haven't used table-valued functions in place of SPs, so I don't know what additional complexity that might offer.