How to get breastfed baby to gain weight?

How old? How much did she weigh at birth and how much does she weigh now?

Some babies are just smaller.

How long are you feeding her? My ped said no less than 15-20 min each side for weight gain. That’s when the fat comes in through the milk. And To wake them if they fall asleep during eating.

Make sure she’s getting enough and does not have a tongue and lip tie. My son did and was not taking enough each feed!

Try pumping for a bit if you can. Monitor how much milk you are producing. Knowing that will help ease the stress of wondering am I giving my baby enough to eat. And also talk to her pediatrician about your concerns. I was monitoring my milk production and thought I wasn’t making enough however that little 2 ounces was more then enough to get my baby where she needed to be.
Lastly breastfeeding can be challenging to keep on an exact schedule but try to start latching her more frequently for shorter amounts of time. Maybe every hour or so for 10 mins. Doing this increased my milk after a day or 2.

Are you eating enough? Try implementing more fats into your diet and protein too. Avocados, nuts, bacon, beef, cheese. My lactation consultant told me my baby wasn’t gaining enough either, but she was only 2 weeks old at the time. She also had a tongue tie that we had clipped when she was 4 or 5 weeks. She’s now 3 months old and has doubled her birth weight. Don’t worry about it too much mama. Foremilk is the first milk that is released which is mostly water, hindmilk comes after and is the fatty milk. Breastfeed only on one side each feed. Switching breasts during a feed could mean baby is only getting foremilk which could result in baby not gaining. Keep breastfeeding on demand, you got this!

I’d try pumping too! Pump for like 30 to 40 mins to make sure she’s getting the hind milk

Is baby not gaining at all or not gaining as expected on a growth chart? Please be aware most growth charts used in the US are based on formula feeding not breastfeeding. Weight gain rates for breastfed babies are lower than formula fed. I believe WHO or UNICEF has charts based on breastfeeding.
Also how many wet and poopy diapers is baby getting in 24 hours?