User's location stops sharing in Family Sharing all the time

I added my wife to Family Sharing in her account on my Macbook. After that I can see her phone's location in my Find My iPhone account, and in Find Friends, and in Messages. She can see me. All works fine.

But then after a period of time, maybe a day or a few days, I can no longer see her location in Find Friends, Find My iPhone or in the Details pane in Messages. She is no longer sharing the location with me. She is still in the Family Sharing group though. I have to login to her account on my Macbook and set it up again. I have done this about 4 times now. I think I will stop using Family Sharing and just use Find Friends because my other friends in Find Friends (but not Family Sharing) do not get disconnected. She says she has not clicked anything to stop sharing her location, so its doing it automatically.

Does anyone know why this may be? On her phone she is using my AppleID for Music/iTunes & App Store and Apps, but I think thats it. She uses her own AppleID for iCloud (and therefore Find My iPhone)

Thanks