The radius of the curve across the fillet is greater than the radius of the edge along which the fillet is modeled.
While modeling a fillet in SolidWorks, the radius you assign to it is its across curvature. The radius of the edge along which a fillet is modeled is its along curvature.
If the across curvature of the face you are trying to recognize is equal to or greater than the along curvature, Featureworks cannot recognize the fillet. This limitation applies when the along curvature and the across curvature are both convex, as shown below, or both concave.
If the across curvature is concave and the along curvature is convex, as shown below, FeatureWorks can recognize the fillet.