Two Mikrotiks wont ping each other - how can I diagnose this?

After rebooting the one that has usermanager running on it I can ping between the two for a few minutes then it stops. Rebooting the client doesn't. Other devices/mikrotiks can ping either mikrotik without issue so there must be something on either of them that is explicitly blocking the other?

Found the cause:

Each mikrotik has a wifi vlan which I'd used the same MAC on ... have done this with 5 mikrotiks already and didn't see this issue after testing each one for a week, mind you there have been new OS updates since then ...

oops. I was hoping to have wifi devices see the same mac address in the air between access points.