Instances receiving different messages

I'm seeing a weird missing message issue. When I run six instances of a Java receiving application (on Windows) and a single C++ sending application (on Linux) not all six are receiving the messages. The number of messages being sent is small and the sender is sending messages to a single machine running all the receivers on. This is weird as I would have thought if a UDP message is lost then all six would fail to receive the message but infact some receive messages that others do not. As mentioned the sender is sending a very small number of messages (~50).

Could this BEST_EFFORT_RELIABILITY be causing this issue? How is it six instances of the same application are losing different messages to eachother? I do not believe there is a problem with the implementation of the interface code as its a simple waitset implementation and its been extensively tested using a Java sender running on the same machine.

Thanks