Help for Bessie

One of my girls Bessie is being bullied. Every evening I can hear her squealing. She is submissive, letting the others dominate her but even when she lies on her back they still attack her. I've checked her and can't see any injuries to speak of as yet, so I'm confused about this behaviour? The other rats she's with (bar Dorothy) are her full sisters who shes never been seperated from, and it's usually them doing this to her and not Dorothy. I am at a loss what to do as its been going on about 3 weeks now. Additionally any time I look in the cage Dorothy, Betty and Elsie are cuddled up together in their rat house, and Bessie is on her own on the top shelf so it seems they either won't let her cuddle with them, or she doesn't want to. Any suggestions? Bessie is a real heart rat and my favourite of the four but no idea how to help her. I am reluctant to seperate her from her sisters though as I feel this would be more damaging.

It could be a short phase, if they're not injuring her, I'd not separate just yet... Could be that if she's bottom dog they are constantly asserting their dominance over her.

xx