In the past I had problems with using the tab key to moving from an edit box to a dropdown combo box to the next object. I could tab from edit box to edit box but as soon as I hit the combo box the tabbing stopped. If the combo box (in properties) is set to be just a "dropdown" combo box type then tabbing will not work. However, if you change the property "type" of the combo box to be a "dropdown list" then tabbing works fine. I found this by accident.