Air Conditioner troubleshoot

Hi All,

I have an ecobee lite 3 thermostat that was installed a few years ago. I noticed when I set my AC temperature to (whether it's 0.5 degrees or 4 degrees) below my house temperature, the AC unit doesn't always run. I am able to turn the fan on and off though.

I also turned off the ecobee+, but doesn't make a difference. The AC unit will sometimes respond, but not always.

Any idea how to troubleshoot this?

Thanks.