View client sends messaging to setup PCoIP UDP display protocol between Horizon Client and assigned resource. This traffic flow occurs from Horizon Client to APM over ssl and proxied from APM to assigned resource over TCP 4172. You should see communication over tcp 4172 from APM to the assigned resource prior to seeing any UDP 4172 traffic. If not, I would check the port is open between APM and the Virtual Desktop / Virtual App networks. Once initial setup occurs you should begin to see UDP 4172 traffic from client to apm and from apm to the assigned resource.
I would also verify the connection servers are not setup to use https/pcoip as APM handles directing clients to the correct ip addresses. Another thought is to run a capture on the client to see if it is sending UDP packets anywhere other then the correct APM IP address.