The above message is very generic and can mean any of the following :
a. the SNMP setting in the device has been turned off
b. the device is in a subnet that LDCM can't access
c. the device is not connected to the network
d. the IP address is being used by another device that is not a printer (it may be a PC, server, etc..)
We need someone to help verify the above possibilities and resolve.