Two of my three WiFi Routers lose connection

I have had one WiFi Router, a Linksys e1200, provide WiFi access for our company without problem for a year or so.  Recently I added two more of the same routers in different areas of the building so we can have a wider range of WiFi connection.  These two recent routers continue to lose connection and therefore WiFi access is not possible until the router is reset.  They work for a day or two, and then lose connection again.  I have tried to change to different channels and other settings for them but nothing I've tried keeps them connected for any length of time.  I never have problems with the original WiFi router.