I would consult your babies doctor on this one but with my two neices who were born small both in the 5lbs range it took them a long time to gain weight. As long as the baby is having atleast 6 wet diaper and 1 dirty in a day then the baby is eating enough.
My newborn baby is having hard time adding weight to her birth weight. She is being nursed but does not seem to gain enough. She is 5 weeks old and weigh 6 lbs and 2 ounces. She was born a little less than 6 lbs.