I have a 5 day old litter of 6 puppies and noticed today that it takes a good 10 minutes for the milk to "let down", there appears to be plenty of milk when it does come, but just wondering if it is normal for it to take this long?

The 2 smaller puppies seem to get tired and give up trying, whereas the bigger 4 keep on pummelling away, as soon as the let down occurs, I plug the 2 small pups back on so that they are not missing out, but never seen this in previous litters.

Might this suggest that mum is not eating sufficiently or something else wrong?

I have never experienced this but if the puppies are plump and growing then it would suggest that there is nothing wrong, maybe your bitch takes time to settle into the feeding and you are doing the right thing by plugging on the smaller ones, it will probably come right as they grow bigger

As with people stress will interfere with the let down response, so make sure that the bitch is settled, there is nothing that worries her like the sound of the other dogs, and over exposed or even over secluded (some want the reassurance of your presence, some prefer to be more private) whelping box location.