DIAGNOSIS: This is often due to dust buildup you can't see on the motor shaft or the inner hub of the blade assembly, creating drag. It can also be a sign of a failing motor bearing.
FIX: Go through a thorough cleaning as described in the cleaning question. Pay special attention to the area where the central blade column attaches to the motor shaft deep inside—this is a major dust accumulation point. Use compressed air at all angles. Also, ensure the fan is on a flat, hard surface. On thick carpet, the base can sag slightly and misalign the internal components, subtly hindering performance.
IF THAT DIDN'T WORK: If cleaning and placement don't restore power, the DC motor itself might be degrading. These fans use brushless DC motors which are generally reliable, but the bearings can wear or the motor control board might not be delivering full power. There's no user fix for this.
HEADS UP: Don't be fooled by a clean exterior grille. The performance killer is the matted dust layer on the actual blades and motor, which requires more aggressive (but careful) cleaning techniques to remove.