My sorority did not have a house, but a friend of mine from a housed sorority tells me that if there was a vacant spot in the house, all sisters who were eligible to live there but chose not to had to pay their share of the rent for the vacant spot (rather than having the rent come out of the chapter's main operating budget). It sounds blunt, but if eligible sisters have to take a hit in the wallet, they may reconsider.
You may want to make up a list of the advantages of living in the house - for example, if room and board is cheaper than living in the dorms, say so. If your house needs renovations, make them. Maybe you can find a way to make it easier for a sister to move back into the dorms if she lives in the house for a semester and decides she really doesn't like it.
We never had this problem, but I hope this helps anyway...
