LO not drinking as recommended

My 8 weeks old LO has been drinking lesser (expressed breastmilk) and is barely hitting 500ml a day (recommended for her weight is 675ml) with each feeding varying between about 70ml to 100ml. She still gets wet diapers and poop daily but the weight gain has slowed (dropped from 50th percentile at birth to 25th percentile for weight and head circumference. Length growth maintained at 75th percentile). In week 7 she actually managed to drink more than 600ml but it has came down since. Any one experiencing the same? Any idea how to get LO to drink more? #firstbaby #1stimemom #advicepls