Could have an object stuck in her Belly Richard,which puts them off food,sometimes they swallow something lets say for example a hose sucker attachment or something similar if they struggle to pass for some time put's them off there food,just a thought..when they get older rays still feed the same,i have 12 year old rays that would eat me if i fell in the tank!!
Gravid females are always top dog's at meal time.
ive got the snails in filters to,millions of them in my filters,never had plants or sand.