How can I help my baby to drink more

My 9 week old daughter only drinks 2-3 oz of formula every 4 hours and she isn’t gaining enough weight. She is on the 2nd centile.

I try and give her more but she just falls asleep after 2-3oz and she doesn’t want anymore. I do try a but she won’t take it. She naps frequently throughout the day so I know she isn’t tired. Plenty of wet nappies and soiled twice a day.

I am just worried about her. She doesn’t have any allergies or suffer from reflux and I know she enjoys the milk she does take but wont drink anymore. I just wanted any necessary advice? Thank you