You cannot set focus to a form if it has any enabled controls - can't say whether or not that includes enabled but hidden. The last control that had the focus is where the focus will go, not to the form itself. Not sure if you have to set focus to a subform control first, then a control on the subform or whether you can do so directly. Regardless, if the subform contains enabled controls it can't take the focus anyway. What is the purpose of trying to set focus to your subform?
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.