I also set my dishwasher to air dry. Once the washer has completed the wash and rinse cycles, I open the door a crack and within an hour or so the dishes are dry and ready to put away. If there are hollow spaces on top of glasses and cups I dry those while they're still in the dishwasher before putting them away.
If you want to use the dry cycle check to see that it's still working by opening the dishwasher while it's in that cycle. You should feel heat and see the coil red hot. Your dishwasher may need a new switch.
Call a repairman and ask over the phone for possible reasons and probable charges for repair.