Low Birth Weight
Sometimes in a litter you will have one puppy just not as big and thrifty as it's siblings.
a.  The rest of the litter just pushes him off.  With this one you can just stay there and make sure he gets on the nipple and hold him there until he fills up.   I do this every two (2) hours for the first 48 hours.  Than stretch it out to three (3) hours for the next 24.  After this time, the puppy has usually gotten strong enough to go on his own.  You just have to keep an eye on him to make sure he is getting filled up.  Sometimes making sure he gets on a nipple only a couple times a day.  You can tell by  this time if he needs more.
b.  The puppy does not seem to attach itself to the nipple.  You get him on and he just falls off again.  Sometimes this is a sign that the puppy is actually dehydrating and does not have enough saliva to get a good attachment.  With this puppy, I would moisten the nipple with mom's milk until it is very wet, than put the puppy on the nipple.  It may take a few tries but it usually works.  If you have a hard time getting the milk out of the nipple, just plain water will work.  Use warm water and drip it onto the nipple making sure it is very wet and place the puppy on the nipple.  Sometimes I even continue doing the dripping while the puppy is nursing to make sure the nipple stays moist so he does not lose the attachment.  I would follow the same schedule that is out lined above until you feel confident that the puppy will do okay on its own.