Make sure your coolant is full, and try bleeding the air out of the system. The switch is a wax-type switch, so the wax has to heat up for the switch to activate. If there was an air bubble, then it might not activate the switch. So get the air out, fill it up with coolant, and fill up the reservoir.
If that doesn't work get a multi-meter and make sure the fans are getting a current when cycle starts.