Have they checked her for tongue tie? You could try different bottle teets as that maybe an issue too.
Just remember that the centile charts are based of formula fed babies. Try not too worry too much as long as baby is putting on weight and seems happy with lots of wet nappies x
My son dropped percentiles but he was born 10lbs9oz, he’s tall and lean (31 inches and 23lbs at 9 months), and has still been gaining consistently just not as rapid to keep up with percentile. If they’re still gaining consistently and developing normally (hitting milestones) I wouldn’t be too concerned. But best to contact your GP or pediatrician.
Hm my baby has been dropping percentile but it's probably more to do with the fact she was born nearly 9lb, but neither I or my husband are particularly tall people. The LC said to not be alarmed if the percentile started to stagnate based on our size the baby's growth would also slow down. Not sure if this could be the case for you?