if the fan isn't running, that's the problem. If you can hear it running, then the problem is that it is not running fast enough, i.e. not moving enough air to engage the sail switch. This can be caused by low battery voltage, or by blockage in the air path. Since a single motor drives both the intake/exhaust fan and the air circulation fan, a blockage in either one causes problems in both.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL