RS485 challenge

I connected seven devices to Sixnet RTU (VT-IPM2M-213D) using RS485 RTU protocol. I observed that when the seven is powered on, one of the devices does not work when it is in the RS485 network with the other devices but it works when it is the only device on the network. What can I do to resolve this challenge?

See the responses listed here: http://control.com/thread/1478866302