Subform Control Lostfocus

I've use a set up that works fine on a form but doesn't when used in a subform. Basically, i have a check box and txt box that won't allow anything to be written in it unless the check box is checked. If it is checked and nothing is inputted in the text box, the lostfocus event for the txt box flags the user. It works fine in a normal form and when it's a subform, the lostfocus event does fire when going to another control in the subform. The problem is that it doesn't fire when going to another control not in the subform. Is there anyway I can get the lost focus event to fire under these circumstances?

Thanks,

scratch