I'm going to make an assumption--namely, that you are communicating Ethernet/IP? I had exactly this problem a while back, and found that the problem was in the Omron configuration, I had the input connection set to Multicast (which is default). I changed it to point-to-point and the problem went away.
Smurrill - Thanks for the tip. I am trying it right now, and I do not see within the Omron where to change that. I know exactly where I would within an A-B, but not Omron ...