
Agree keep plugging them in every so often if they have been off Mum for half hour, the extra nursing will bring the milk in and pups will start to regain/gain.
I'd only intervene if a pup wasn't able to suckle well, by making sure to wake it often and ensure it didn't get knocked off.
A normal pup should have enough reserves, to keep it nursing and the energy from the colostrum, and what it gets from the bitch..