When I had a little runt in a family of four I used to let her spend the day with her feathered family but every morning I would bring her in for some extra nutrition and attention from me. Sweetie was killed by a raccoon one night but before that she grew up to be the same size as her sibs and was doing just as well. She was not a crossbeak but for some reason she was behind the others in growth.I have 1 with a slight cross beak. She doesn't gain weight as well with the general population, but she was lonely the last time I separated her. She does eat and drink but doesn't always get the same amount of food b/c she is pushed away. Not sure what to do.
Try some extra nutrition once or even twice a day with her but do let her spend the majority of the time with her feathered friends. That is my recommendation.