I do something like the following when I'm debugging subreports:
1. If you're using the report header section of the subreport, create a new one, otherwise use the one that's there.
2. Put the parameters that are created for the links in this header so that you can see the values that are being passed to the subreport.
3. Create a new details section and put a few fields from your data in it.
4. If you've modified the selection criteria to contain more than just the links from the main report, delete everything but the links.
5. Run the subreport only - providing valid values for the linking prompts.
6. If you deleted anything in step 4, add the addition criteria back in, one statement at a time, refreshing the subreport for each statement to see if that's what's causing your problem.
If you see data from the new sections but not from what was originally in the subreport, then you've got suppression formulas set up that are suppressing the original sections of the subreport.
-Dell