You can view traffic in the log files on the Sonic Wall - you should be able to see if you are getting packets - either from the IP address of the other 2 sites or on the UDP ports you are using.

I have also seen issues with Sonic Walls - everything was configured right on them but they didn't forward the traffic properly. Eventually Sonic Wall tech support resolved it.

Have ESI tech support dial in and verify your programming is correct as well. Also have them make sure all of your licenses show as active.