One of 6 airport express no longer accepts music from old Mirror Door. Other MACs work fine.

I have been using an old mirror door to stream music around my home and garage. It works well except one airport express- a newer one, stopped accepting the multiple speaker check box. My other iMacs and powerbook with newer iTunes do connect to all six express units. What could cause this? It shows up but the check box doesn't stick- Sometimes it will flash the connection window briefly but it never connects.