It will be to do with the settings you set when you listed the item- as soupy said maybe you said no-one with 0 feedback, or people in certain countries (I don't post to Italy, as the postal service is horrific, and items have gone astray previously).
This is all to do with settings listing those people you will NOT accept bids from.
These are usually places that are unreliable to send to, people who may never have bought/sold on ebay before and therefore have no history, people who have received negative feedback etc etc.
You need to find out why her bid is being refused/stopped - is she in a country that you have set as being one you will not send to?
I think, if I recall correctly, that ebay have a default setting when you sell stuff so you may not be aware of setting any of these perameters. Go into revise listing, go to the buyers requirements (somewhere near the price/postage area) and reset your list.
I have had this happen a couple of times, both times they have exceded the number of unpaid item disputes. Both times I have replied very politely explaining that my activity log indicates that they have too many unpaid item strikes. Both times they have responsed with loads of abuse and I have been so glad I didn't make an exception for them.
Have a look for the reason here Home > My eBay > My Account > Preferences > Buyer Requirements Activity Log