Is IMC managing the default GW for that client? If yes, what sort of device is it?
IMC needs to be able to get the IP/MAC mapping from somewhere, and it would normally do this from the default GW, by polling its ARP cache. This is obviously not a fool-proof method. It might be that IMC can get the MAC from the CAM table of the switch the device is attached to, but doesn't have an IP->MAC mapping from any other device.