R & R question on inventory

here's a question i have had for as long as i've been in the dealer parts department....
why is it when i go to bill a part out on an invoice or on a repair order, sometimes the quantity on hand shows zero? i have the part on the shelf, and if i use the "Q" function to look up a part and it shows a quantity of one.
any ideas?
why is it when i go to bill a part out on an invoice or on a repair order, sometimes the quantity on hand shows zero? i have the part on the shelf, and if i use the "Q" function to look up a part and it shows a quantity of one.
any ideas?