Is it normal during teething for a pup’s drive for sack or tug to diminish to a level where it is almost nonexistent?

Is it normal during teething for a pup's drive for sack or puppy tug to diminish to a level where they are almost nonexistent? My 5 month old GSD who normally acts like a little alligator, now because he is teething, is only content to retrieve a tennis ball and track. He has mo interest at all at training with the helper or at home with me in sack work. Doesn't even get worked up watching the other dogs work in Protection like he did before he started teething. He used to bark like a madman and pull on the leash to get to the helper. I reviewed my vast library of your videos and though you mention not to let pup take any bites during teething, you need to continue to motivate the drives. At this point in time my pup has no interest in the sack. Any ideas not mentioned in the videos I can use to bring back this drive?Not to veer from this subject but I would like to commend you on the dogs you produce. I train with the breeder of my pup, Landschaft Kennels in Marengo Ill. Another member of the club is Steve Billimack who owns one of your pups, a sable bitch named Elke (I believe her registered name is Dina vom Leerburg.) She is out of your past stud Chucho. This little bitch is awesome and consistently out performs the other club dogs, including the Titled imports. This is a real credit to your breeding program. In April she took High in Trial at the AWDF Nationals and is now being pointed to the USA Nationals.

Sue