Take the MRF to a different room, away from the equipment and test it there. If the receiver or cable box is throwing out too much RF you will be far enough away and should see the status light illuminate and the emitter as well.

If it works there but not when you put it back in the equipment stack, start unplugging devices one at a time until it works. I just had a customer owned Denon receiver last Monday that was the cause of my RF not working with a MX-900/MRF-260 setup.

Don't forget that setting the MRF to 0 will help you in troubleshooting RF issues. Do you see the status light flash at all? If so, does it flicker at all after you release the button or does it cut off immediately when you release the button?

Hope this helps.