The first thing to check for is if there air is working, the teat wand is ran of the red air compressor in the pump room. make sure the compressor is running.

If the air is working check there is power to the system, there are red lights on the 3 Senors above the wand. (if there is no power contact Alex). 

If there is power and air and it is still not spraying the cows then there will most likely be a issue with one of the 3 blue sensors. The Teat sprayer needs all 3 sensors to read before it works, so if one is not reading what it is supposed to it won't work.

Possums climbing in the dairy will be the main cause of sensors being knocked. cow shit can also cover the sensors giving them a false reading.

This sensor is right at the top, it reads the bail tags that are on top of the platform.

If the sensor cannot see this then it wont spray. it has a red laser that points at the bail tag. If you cant see the tag then the sensor might be out of alignment.

This is the retention bar sencor the teat wand wont spray if the bar is down. make sure this is pointing to the retention bar. 

This is the last sensor, this reads if there is a cow in the bail. make sure this is pointing at the platform.

If there is still nothing, press manual and purge.

if nothing is working turn it on and off again.