Not gaining weight

slpatschool

E is 3 weeks old and as of her last weight check a few days ago she is still almost a pound below birth weight she lost 10 of birthweight. She has a sufficient amount of poop and pee diapers, is well hydrated, and feeds every 2 to 3 hours for an hour. I use a nipple shield but I always check to make sure there is milk pooled in the shield and I check her mouth afterward to make sure she is actually getting milk and not just sucking.
I also pump once a day and get about 2 oz so I don't think I have a low supply.
Her doctor is pushing me to supplement with formula but I've resisted so far.
Have you ever heard of an infant taking this long to get back to birthweight? Any thoughts on what more I can do? Should I supplement if she hasn't gained by her next weight check in 4 days?