Just a thought to  bear in mind when nesting subforms.
Friday I  spent about 3 hours troubleshooting a nested subform that behaved like a single, rather than a continuous form. I thought the form may have become corrupted, so I pulled a working basic backup copy and began adding elements to the new form footer and testing after each new element.  Eventually the new form did the nearly the same thing as the old one. I dug a little deeper and found that Access forms have a maximum height of 22 inches.  So the more detail I added to the footer, the fewer rows the detail could show, until it APPEARED to become a single form.  Bottom line, I had too much detail in the footer.