I had a gree 3 zone mini split installed in 2016. One 12k unit and two 9k units. One of the 9k units will cool the room to the desired temperature when powered on but once the temperature is reached it won't keep it at that temperature. If I set it to say 68 degrees for bedtime, it will cool to 68 but after it reaches that temp it will only circulate the air in the room but will not cool it down again. Both of the other two units function properly so I'm not thinking it's a refrigerant problem, could it be a malfunctioning board in that individual air handler itself? Thanks for any direction y'all could give.