2005 Hyundai Elantra AC doesn't always turn on

Air Conditioning problem
2005 Hyundai Elantra Manual 41000 miles

The AC in my 2005 Hyundai Elantra doesn't always turn on. Probably 7 times out of 10 when I turn the car on and flip on the AC nothing happens. If I have the outside vents open the air flowing in will be cold. Sometimes it will kick on when I'm driving but usually not. But for the times when the AC will turn on it works great. The air blows cold, works on all settings and never kicks out when I'm driving.