How much weight should 6-week old kittens gain per week?

I want to be sure that my foster kittens are growing like they should. I'm a little concerned that one of them only gained 1.25 ounce in the past week while the other two have gained more. I was just wondering what the normal average is. I'm not completely sure how old they are but I believe they are about six weeks based on their teeth development.

The minimum weight gain for kittens about .4 ounces a day.  It should probably be more as the kitten gets older.   May I ask what you are feeding them?