1. The first thought should not be the motor lubrication, resulting in after fan mounted on the motor can not drag. You remove the fan, plug in the power, if the shaft can rotate, indicating that the problem, if not that is the other problem.
2. Your fan is not overheating, motor overheating can cause deterioration of the motor with a load capacity, low active power of the motor, making it impossible to drive the load.
3. Next, you have to consider how long the fans used, the motor loss, the motor bushings wear, it is easy to burn. Motor with a long time, it will also make the internal resistance of the motor becomes large, with a load capacity becomes low, making it impossible to drive the blades rotate.
4. There is a start capacitor smaller capacity, although reads those parameters on the nameplate, but a long time, the capacitance is reduced, leading to the starting torque of the motor becomes small, can not drive the load. This simple, remove the capacitors, go for a new, same model should be no problem. But preferably for a 20% larger than the original, you will get better results.