One cause could be wired and wireless interfaces on the router are causing your system to be registered with a different internal IP, which would result in port-forwarding settings being available for one, but not the other. You could try manually assigning the system to the IP (and other settings) instead of letting the router assign it, but I don't recommend that much, or fiddle with your router's settings to always assign the computer with a specific mac address to that IP... if the router provides that option.

You should Google more details specific to the router model you're using.