I’m running a server 2008 R2 box. Having DNS service running on the same machine as WDS causes DNS to steal the ports that TFTP needs.
Run regedit (with Admin access obviously).
Right-click UdpPortPolicy, and click Modify. In the Value data box, type 0, and then click OK. Exit Registry Editor and then restart the Windows Deployment Services service.
Worked a treat for me.