After doing a bit of digging, I found that I have the same problem. Just didn't think nothing of it until trying to output the reports automatically to email as an attachment.
Here's what you need to do.
1. Save the forms as a report.
2. Go to the main form and locate the subforms.
3. Change the Source object under the Data tab to the form, saved as a report
Works like a charm.
My ref is Microsoft Knowledge Base Article - 207489. It'll give you step by step procedures. Just make sure you save the correct form as report and select the right report as the source object for the subform. If you have any questions, let me know.
Myk