DHCP/PXEBoot via VXLAN

Hello,

I've got a 100e in our DC and a 40f CME device connected via an L2 over L3 IPsec tunnel using VXLAN as the encapsulation.

I've used these instructions

https://kb.fortinet.com/kb/documentLink.do?externalID=FD40170

both phase1 and phase2 come up and I can ping between devices at either end of the tunnel, however I can't get the remote device to obtain a DHCP least which would then lead to the device PXEbooting from the host in the DC.

I've got the policies fully open on both firewalls between the physical interfaces connected and the VXLAN tunnels, so it's odd that ping works, yet DHCP doesn't.

Could someone give me any advice on how to troubleshoot? There's not a great deal of information on the internet...

thanks,

Ben