We have mikrotik running on Xen. Everything has been running smooth almost a year now, but I have noticed the last couple of months, on 3 client devices now, where mikrotik stops responding to the client device. The latest one happened today.

I can do a ping from one of the affected devices and run the packet sniffing tool on MT and I can see the incoming packets, but MT does not respond to them at all.
No firewall changes have been made in months and I double checked to see if anything was not perhaps added magically.

The devices are being MAC authenticated, switching to PPPoE makes no difference. I have resorted to a backup gateway running pfsense, but I need them going through MT to account for traffic being used.
I'm running Routeros 4.3

Any ideas why this is happening?

Well I still don't know why this is happening or what caused it, I have however manage to figure (after a suggestion from a colleague) if I change the ip address statically on the client to something else, it works fine again.

Very strange, but glad I have a solution.