Dynamic DNS host not resolving in IPSEC tunnel

I have a Fortigate 100D running 5.2.9, with a couple of Mikrotik RB952s connecting via IPSEC tunnels. Both tunnels are set up in the same way, with the IPSEC tunnel Remote Gateway set as Dynamic DNS. One of the tunnels works perfectly, with the DDNS hostname resolving to the new IP every time it changes. The other always gets stuck on the existing IP, and does not update when the IP changes. The dynamic DNS host is being correctly updated and resolves from the CLI, but not the VPN.

Any advise would be much appreciated.

Thanks,

Patrick