Does anyone know how to get my puppies to be weaned? They will be 6 weeks old.?

I have tried several times to get them to eat. I moisten puppy chow and mash it up with broth from the food. And they won't eat it. I have tried separating them from the mom for awhile still they don't want to eat. This is the mom's first litter and she is very protected of them and doesn't want to leave them for very long. I don't know what else I can do to get them to eat. The puppies have already been sold. And I wanted to know if this is normal that they don't want to eat yet. The last litter my other female had; the puppies were eating at 4 and 5 weeks of age and the new owners came and got them when they were 6 weeks old. But these puppies are totally different. They just don't want to eat the food. Any help on how to get them to eat will be greatly appreciated. I breed dogs and the new owners are really anxious and want to get their puppies as soon as possible. Again thanks to everyone who answers this question!