When is your wedding? If you have time, I would suggest you pull this MOH aside and be up front with her. I would say, it does not appear that you are really interested in being my maid of honor; is that true? I am fine and understand if that is the situation. I really need someone to be my MOH that will be there for me as I need the help. If you cannot, I understand, I just ask you to step down so I can request this of someone that is a better spot right now to help me.
Be kind but honest. Maybe this is her way of getting out of it, by not really performing what is usually expected from a MOH.
You really do not want this to impact your wedding.