First, check the temperature at the vents with the temperatures both set to "LO" but "EC" illuminated. They should both blow warm, but the same temperature. If so, go on. If the temperatures differ, you need to troubleshoot the heater.

Next, ensure that you have the correct refrigerant charge. Being a bit low on refrigerant can cause this. However, if the refrigerant level is correct and you still have warm left-side vents, there is likely a restriction in the system.

You can approximate the refrigerant level check by connecting gauges and reading the pressure charts for the AC system, but that really is very approximate. However, if your high side pressure is way below specs when you get the warm air out of the left side, the problem is almost certainly a low refrigerant charge.

The only way to be sure of the refrigerant charge is to reclaim what is in there and recharge by weight.