Windows Deployment Services multicast with vLANs

I having trouble allowing WDS multicast to reach different vLANS on my Fortigate. Unicast passes through the vLANs so I know my WDS server is reachable and working, however when I try to join the mulitcast stream it simply tries, fails and then rolls over to unicast and continues to image the machine. Multicast within the same vLAN as the server works. 

I've enabled multicast forwarding from the source port to the vLAN interface I'm trying to image. Allowing multicast traffic from the specific server IP source to "all" IP addresses as the destination. 

I'm sure I'm missing something, but don't know what.

Thanks!