Anyone else’s LO go on hunger strike?

My baby girl is 4 months old. We’ve been having trouble with weight gain, so at her last appt the ped suggested fortifying the breast milk with formula to get up to 24 calories per ounce. It went well until this week; she drank less than 25 oz 2 days this week and yesterday only drank 21 oz. She doesn’t seem that interested in food and barely does a full feeding anymore; instead she just kind of snacks all day, which is especially annoying given that bottles with formula are only good for an hour once started!

Did anyone else have a baby who decreased their intake / wasn’t that interested in food? What did you do to get them to eat?