You say that both forms use the identical subform. Is that using the SAME subform or is "identical" simply a copy?
Is the sub-form the only part that cannot be updated, or is the parent having the miscreant subform also non-updateable at the parent level?
Are both form/subform pairs using the same parent/child linking fields?
Is there a filter on one of the parent forms that isn't on the other parent?
Or do both parents have the same .RecordSource?
Do you have code on the subform's events that might be context-sensitive?
You are looking for differences but they have to be somehow related to the parent if you are using the same form for both parent forms. But if you are working with identical copies, then it is possible that the difference is that the identical copy isn't identical. Fill us in on that level of structural detail.