I would get them checked by the vet myself to ensure that there's nothing there to cause a problem. They could be just greedy pups but for all of them to be doing it is a bit worrying. Hope it's nothing.
I would just get them a quick check over the vet can look in the mouths with a little torch but I have hd nearly whole litters doing it. I got the freight of my life when I first saw it.
I would still always say if it worries you get them checked.