Refreshing subform data

My database contains a main form that has two layers of sub-forms (a parent, child, and grandchild). The “parent” form is linked to the child via a primary key, let’s call it “serialNum”. The “child” form is then linked to the “grandchild” form via a different primary key, let’s call it “modelNum”.

The problem that I’m having is that when the main form is refreshed (by calling Me.Refresh) the grandchild form is reset back to the first “modelNum” in the recordset. This does not happen in the child form...

Any idea why this might be happening in the “grandchild” form? The call to "me.refresh" is what takes "modelNum" back to the first record -- there aren’t any other triggers that are causing this.