A/C intermittently won't cool
Your A/C system, your zone controller, and/or your thermostat could have a time delay to prevent the system from turning on and off too often.  Short cycling the compressor can be bad for it, so the controllers typically enforce some kind of delay.  

The actual delay will vary depending on the controller, so you'd have to get model numbers, etc, and research which device has which delay to figure out exactly what's causing it in your system.  There can also be a delay built in before switching from cooling to heating, but that's not what you're running into here.