Have you verified the monovalve is actually closing when the AC should be on? If you run the AC and unplug the connector on the top of the monovalve it will open. If you then reconnect the plug you should hear/feel it click shut. Make sure the electical connections on the valve are clean.

Also, is the AC compressor clutch still engaged when the AC should be on, and is the top AC hose cold? That may help tell you if the AC is working and just being overpowered by the heat.