The following requirements apply for the recognition of surfaces for milling features.
Only straight walled-surfaces are recognized. If a single surface also contains the bottom radius or a chamfer it will not be recognized.
Tapered surfaces are not recognized.
The current setup must be oriented so that the vertical walls of feature surfaces are parallel to the Z-axis.
You can include surfaces other than straight walls and they will be used to calculate the depth of the feature. These surfaces will not be used in determining the feature shape.