Finding a defective ostomy product is no fun. More often than not it tends to be a pouch whose leak does not show up until you have spent time and effort to get it installed and the contents have soiled your clothing.
When possible, change your appliance and save the defective product. Using a felt-tip pen, mark the area where the problem is (clean the pouch of course first); use circles, arrows, etc. to mark the problem area. Put the clean item in a plastic zip-lock bag, attach a note explaining the problem, and mail it directly to the manufacturer. (Postal Authorities will be most appreciative if you emptied the pouch before mailing, no matter how much you'd like to "get even" with the manufacturer, also the postage will be less.)
It is normally not a good idea to return a defective product to the place where you bought it. The vendor may not welcome you with open arms--he has better things to do--and may find it cheaper to just throw the item away or keep it on the shelf for months until enough items have accumulated to make a return shipment to the manufacturer cost effective. In the meantime, the manufacturer may continue sending out defective items without knowing it. For speedy action and to help prevent other ostomates from running into the same problem in the future, you're better off going to the manufacturer directly. Remember, most manufacturers want to know if there is a problem with a particular product.