Strange Problem with Samsung Triangle AC

I'm having an odd problem with a Samsung AC unit.  This is an 18000 BTU 'triangle' design - standard, not an inverter model.

The unit will turn on, using the remote control, from right across the bedroom, with no issues.  Range is maybe 6m.  However, getting the thing to turn off again is another story.  I have to use the remote really close up - less than 1m and even then it can take several attempts to turn the thing off!  I think that it's always been a bit insensitive in this respect but seems to be getting worse.

I don't understand this - why the difference between ON and OFF sensitivities?

Not a remote controller fault - I have a spare controller and the behaviour is exactly the same.  Not a simple comms fault either or I wouldn't be able to turn the unit on in the first place.  The unit has been serviced regularly.  It's almost as though the design of the unit is obstructing the line of sight between the remote and the receiver when the swing vane is open.  Doesn't make much sense really and the unit operates normally otherwise.

Any thoughts?