Another possible issue (at least on the X850B) is that the IR input jack can allow the cable to go in a little too deep on some units. Such that when firmly pushed in, no commands will work. But, if you stop the cable from going in all the way (like pad the connector with a thin plastic washer), the cable still holds firm in the jack, but IR actually works.

Took me a while to figure this one out! Wasn't a cable issue either, and I did locate other reports of the same issue.