Try this, in the LostFocus event for txtcomments put code to setfocus to your customer field on the main form. i think you will have to use Form_formname.fieldname.SetFocus since it is on different form.
Depending on how you've got your subform set up, could it be that when you are hitting 'Tab' from the last field in your subform that it is moving to the next record? Try setting the subform's "Cycle" property to current record and see what happens. Just stabbing in the dark here - haven't tried it.