My name is ***** ***** I’ve been involved professionally with dogs in the health and behavioral fields for over 18 years. It will be my pleasure to work with you today. I have worked with schutzhund dogs and trials. I understand the question here. The first thing to check is to make sure your dog does not have any mouth issues. That will require a vet visit. A broken tooth, or other dental issue can lead to a shallow bite. That is the first thing to rule out.
When there is no medical issue present, then the dog is often under stress which is causing the dog to not bite fully. In some cases, the dog might perceive the helper as a threat and thus not be able to focus fully on the sleeve and is more hesitant about that initial bite. This can also come from having an inexperienced helper who moves to fast on the Perceived threat stage of bite work. Often you can get a good bite if the helper is turned slightly away from the dog or moving away. This causes the dog to have more confidence and less stress and makes their bite fuller on first strike. If this does help him, you can work with him on this type of bite to get the firm full bite you want initially. Once he is doing well, sneak a frontal bite in. Even if he bites full the first time then, switch back to the other type of bite work or practice a different exercise to keep the training positive and have him succeed before again turning back to a frontal bite.
Another cause is often us, the handler. We can get a bit carried away encouraging and praising that we break the dogs concentration on the sleeve itself which can lead to a poor bite. It can even be a combination of the two factors. You might switch to a different helper that is less confrontational with the dog and see if he does better with the bite. You might also step back and practice some tug of war exercises to help get a good full bite going again. I did find a schutzhund page that describes the issue better than I can.
Generally my schutzhund dogs haven't had too much of a problem with bite work, but I have been around some and often a change in helper and attitude of the helper goes a long way to correcting the situation.
I hope this information is helpful to you. If you need more information or clarification, please reply and I'll get back to you as soon as possible. If you are satisfied, please take the opportunity to rate.