There are a couple of reasons as to why this might be happening. See below for a couple if things to check:
- Remove the grill cover and give your blower and motor a good cleaning. It is possible that there is a build up of dust and debris that is preventing efficient function of the unit.
- After cleaning the unit, try to rotate the blower to ensure it rotates freely. If it does, that is normal.
- Be sure to verify that there is current from the switch to the motor.
If none of these solve the problem, it is likely a failed blower motor, which would require replacement of the blower motor to resolve.