You may have a "runt of the litter." You may need to intervene if this puppy is nursing, having difficulties with accessing the mother, or if it appears weaker than the others. You may also need to bottle feed if the puppy is struggling. You can feed with a puppy bottle using puppy milk replacer. Avoid human formulas.
If the puppy has a hard time latching onto the bottle, you can try to rub some honey on its gums in hopes of spiking her glucose, and increasing her energy so that a few minutes later she'll hopefully have a stronger suckling reflex.