There can be several reasons for this. First, start by checking the snow sensor supply voltage between pins 2 and 3 of the class two terminal blocks. It should be close to 24 VDC (the label on some unit incorrectly say 24VAC). If there is no voltage then check the fuse on the board behind the terminals.
If you have already simulated a sensor’s call for heat and the controller is not suspect then examine the wiring to the sensor, damaged wires can cause loss of signal from the sensor or power to the sensor.
Lastly, the snow sensors must see both a temperature below 38F and moisture on the moisture grid. High wind conditions can keep snow from accumulating on the moisture grid so consider how the weather is affecting the snow sensor. High accumulations can bury the sensor and allow the moisture sensors to melt a cave in the snow, which will not allow snow to touch the grid. This igloo effect will render the sensor inoperative. This effect can be reduced by the use of a moisture cup on GIT sensors and by placing CIT and LCD sensors at a slight angle to allow gravity to help remove excess snow.