There are a couple of things than can relate to a fan not spinning:
1) The fan is being instructed to spin at a speed other than 100%. Use the fan at 100% for the most consistent results. Sometimes a fan has trouble getting started if it's instructed to start moving but is not given full power.
2) Over tightened screws holding the fan to the Y arm assembly. Sometimes it is possible to push the blades of the fan and it will start to spin. Other times it will be very tight to move by hand. Loosen the M3 screws so the fan turns easily by hand
3) Fan wired incorrectly. In this case the fan may not spin or may spin backwards. Make sure that the fan is wired correctly keeping the colors matched where possible red to red, black to black. If using extension cables make sure that no cables have been crossed and it is plugged into the Printrboard correctly.