P9ing routing differently

[DaveA67]

Hi
I have an Opnsense FW in the cloud with 3 VPNs to remote sites.
2 of the VPNs are fine but one shows odd behaviour even thougnthe settings are the same as the other 2.
The VPN drops out on a regular basis is one issue but a more strange issue for me is that I try to test whether the VPN is up by pinging a piece of equipment at the other end (a PBX). The PBX will respond to a ping.
If I try to acces the PBX config which uses a range of ports, it works fine!
Last week the VPN was up and working and pinging OK
This morning it was down so restated it and it's not poinging.
A tracert from a PC alsdo in the cloud tries to route the ping out over the WAN on the opnsense.
That same PC is able to access the PBX management.

Any ideas as I'm stumped!

Cheers