A couple of things to check out. First how many tits does the bitch have? Usually bigger dogs such as GSD's have more than 5. Since mom is only nursing five it could be that they are constantly going to the same tits. You need to make sure all of them are getting used. The lower tits around the abdomen usually produce the most milk while the ones closer to the chest produce less.

Second, check mamma for Mastitis. This is a hardening of milk around the tits. You will feel the hardness when you touch the tits. It is kind of like lumpy hard cottage cheese. If any of the tits have this feeling then she has Mastitis in that tit. Put a bandage or something over it when the pups nurse so they can't get to it. You do not want them nursing from this tit until it is cleared up. This usually happens in the first week or so after the whelp because the bitch is producing too much milk and the babies don't have the sucking power to drain her dry or they are not sucking off that tit.

This condition is very painful to the bitch and needs immediate attention in two ways. First, you need to get her to the vet for antibiotics. Second, you need to work the hardening to break it loose. To do this apply warm compresses and massage the lump to break it free. This may take hours depending on the seriousness. You need to make sure to keep the compresses warm and keep massaging. It helps to have someone else around to help in this process since it does take a while and you will need to keep applying warm compresses. While this is time consuming it is much better than having to hand feed 5 nursing pups for weeks. They should not nurse off an infected tit.

Let me know if this information helped. In any case though you should let the vet know what is going on. This is your definite source of information and they can see first hand what is going on.