
Make sure they check her heart especially, as poor weight gain ina pup can be a sign of a heart defect. At six weeks even in a big litter I would expect one of my pups (that are 20 - 23kg as adults) to weigh at least 6 pounds at six weeks, and often nearer 8 pounds.
If it is poor rearing I also think she may need worming, and at any rate would be due t be wormed at 7 or 8 weeks depending on if or when last done.
If she is active and playful, and eating well (at least 4 meals, or 6 if she will eat them until 8 weeks) and hopefuly she will make up the lost gorund.