Subform no longer displays after upgrade to Access 2007

I have a database with a form (Main) that has 2 separate subforms within it, I am currently using and developing in Access 2003 but testing it with Access 2007 and one of those subforms does not work when in Access 2007.  I do not recieve any errrors.  The main form does open up to a acNewRec.

Things I have done so far to try and resolve;
- removed all vb code from on load section of the subform - no change
- commented out the "AcRecNew" code in main form on load section - no change
- verified the master and child links are still correct
- tried changing the data entry setting on the subform to yes or no - no change either setting
- tried recreating same subform within Access 2007 - same issue

A little backround as what I am doing I have a database that tracks Root Cause Analysis (Problems) within the company the subform that does not work in 2007 is the form that allows one to enter in "contributing factors" of the "primary cause"  the subform that works allows one to enter in "corrective actions" taken to resolve the problem.  These are very similar in their nature lost as to why one works and the other doesn't.

As a result of this subform not loading, when I try and retrieve a record to edit from a combo box on the main form I get  "error 2467 - The expression you entered refers to an object that is closed or doesn't exist"

I am usually pretty good at figuring these kinda issues out myself but not this one, even though the solution is probably going to be simple and I will want to smack myself once it is revealed!

Thanks,