The remote was working fine for a year then it mysteriously stopped operating my JVC X9 projector (it may have co-incided with a Harmony software update?).

The projector is the only device not in a cabinet. The remote does not power it on. The projector's original remote works and I've tried relearning the 'power on' button from that remote but to no avail. The 'fix it yourself' does not work either. It never will switch it on.

If the projector is on then the Harmony remote can switch it off using the 'fix it' button. But that is all.

Very frustrating. Advice welcomed.

Full disclosure: I don't use Harmony.

Are you using the Harmony radio hub for the controlling the units inside the cabinet? Look into the possibility that OFF is using a direct IR command emitted from the remote, but ON is using the radio hub. If this is the case the projector ON command is emitted inside the cabinet.

By the way, many phone and pad cameras can detect IR emissions. If your camera can display IR, you can verify were IR commands originate.

It always pays to check the simple stuff too. Make sure that the IR path to the projector is sill unimpeded. If the IR optical path is on the edge of working or not, sometimes one command will work OK while another command will not work.

Does your projector remote offer both ON and OFF buttons or is this a POWER button? POWER buttons can be difficult to program because the ON/OFF state "toggles" each time you press the POWER button. With this toggle scheme one cannot guarantee what will happen if a POWER command is issued. The Harmony "fix it yourself" simply sequences through a bunch of POWER commands until all of the units are ON or OFF. This is why the remote asks "is it working" after each attempt to fix things.

This is a long shot, but it is possible that the command being sent to the projector has a repeat that is spaced too far from the first command and the projector is interpreting the command as POWER (delay) POWER -- essentially a null command. Many remote command schemes require receiving repeated commands within a short period as a defense against false triggers. If the repeat spacing is too long, the transmission is interpreted as separate commands. Why would this suddenly become an issue? A software update to the Harmony or projector could have changed the timing and a command that was formerly on the edge has fallen beyond the edge and stopped working.