An invisible fence collar delivers shocks through two prongs that need to be in close contact with the skin. Manufacturers typically recommend that these collars be removed regularly to prevent skin injury, but even so, dogs have been known to develop nasty wounds and infections. Longhair breeds are at an especially high risk.
Change the metal prongs on the receiver. Most receivers come with two sets of prongs: short prongs for short-haired dogs, and long prongs for dogs with long or thick fur. Be sure the prongs are touching the dog's neck. Use the long prongs if your dog is not being shocked when he runs past the invisible fence. Thick or long fur might be impeding the receiver from working properly. 2b1af7f3a8