When referencing controls on sub forms, you must refer to the sub form control name as defined in the parent form and not it's source name.
In the attached example, there is a main form named frmCustomers with a sub form control named Orders. This sub form has as it's source, the form named subfrmOrders. By selecting the sub form (SubFormExample_01.gif, Item A), and viewing the properties, you can see the control can have be named something other than the default name that Access assigns it, which defaults to the name of the form itself (SubFormExample_01.gif, Item B). In this example the name of the sub form control has been changed to 'Orders' from the default that Access assigned it; 'subfrmOrders'.
In this example, we want to refer to the text box 'PurchaseOrderNumber' on the sub form. SubFormExample_02.gif, Item C and D show how to make sure you have the correct name to refer to as you may also rename this control.
SubFormExample_03.gif is a screenshot of the final form. And SubFormExample_04.gif item D shows how to refer to the text box named 'PurchaseOrderNumber' on the sub form.
Hope this helps...
Ken