Openvpn clients work on most WiFi networks but not a few

[pdp11]

I have set up openvpn at home (under ipfire). This has worked for several years. I have used windows, Linux, Android and iOS clients.

Very occasionally (<5% of the time, estimate) when I'm travelling and using WiFi, I can connect to my home VPN server but can't use the internet.

I can resolve and ping an address. Traceroute shows I'm going via my VPN server.

Does anyone know why this might happen? What's the solution or how do I diagnose the issue?

Thanks