This can occur for several reasons, such as inadequate nourishment during pregnancy, large litter, or because it is the queen's first litter. You have weaklings that are crowded out of the nipples by the other kittens. Sometimes kittens that have been seemingly nursing for days will die of starvation. Look at their bellies.
Some nipples produce more milk than others.
Unfortunately, kittens develop a preference for a particular nipple in the first few days of life - you can try to switch a kitten to a more productive nipple, but he might return to the original one. Weigh them every day. If the kittens aren't getting enough nutrition, they may not survive without your help. You will need to supplementally feed them.