If one of your babies doesn't seem to be gaining, you can remove the other pups for a few minutes and give it time to nurse without competition. To help Mom make milk you can try this whelping pudding recipe. Here's a link to it...
http://www.yorkietalk.com/forums/1293603-post7.html You can also try giving her yogurt or cottage cheese to help provide her more calcium. When you take the babies in for their tail docking you can bring Mom in too for an oxytoxin shot. It will help to clean her out and stimulate milk production. When you try to check Mom for milk, squeeze one of the rear teats. Sometimes you have to make a pulling motion as you squeeze much like a baby nursing. Your pups should all be gaining now but sometimes one will be slower to gain. Keep checking the weight and check for dehydration. To check for that just pull up on the skin along the back and see if it springs back down quickly. You can compare a pup you know is thriving to any one you suspect may not be eating enough to get an idea.