why is my litter of pups not cutting teeth by 6 weeks?

I adopted a older yorkie dog and had no idea that she was pregnant when I got her. I think that is why the breeder was in such a hurry to get rid of her. She must have known that she had a crossbred with what I believe was a shih-tzu . The pups will be 6 weeks old in just a few days. One pup got his teeth when he was 4weeks old. The rest have 2 back teeth and ridges on their gums like they will be cutting through their gum any time. One has 2 back teeth and no ridges to even suggest he is going to cut any time. The vet treated them for worms a few weeks ago and gave them a clean bill of health, so I am stumped.

It just takes time for their teeth to grow in, is all.

I can only guess that theyre just taking a little bit longer for these pups teeth to come in, or some may not come in at all. I'd talk to your vet about feed with these pups, see what they think.