She sounds like a very sweet Yorkie!
Maybe she's scared...I'm betting she's very confused. Do you know how many times she has been rehomed...maybe she is still living with the fear of being taken out of yet, another home. It could be that she hasn't learn how to trust and feel comfortable in a home...she could possibly be waiting to make another move. It might take her longer to bond because of what she may have been through. I would give her time....plenty of time so that she can feel like she's finally in her forever home.