Family premium - problems with accounts listening at same time

[Loucolley]

I have purchased family premium, and invited my husband and daughters and then they accepted the invite and have their own accounts under my umbrella. Both my daughters have an amazon echo and even though they are both playing playlists that they have downloaded they can only play from one device at a time despite being logged on to their own accounts, and their echos stop playing, can anyone help with the set up please?

Losing the will......!,