Masks are not created for features with no symbol, regardless of method.
For complex marker symbols, the Convex Hull method will fail to generate a shape when Margin is equal to or greater than a certain value. This value varies based on the reference scale and the units of measurement. In this event, no result is created for the mask feature.
Ensure that features are symbolized with a visible symbol.
A smaller Margin value is needed to achieve a result. Temporarily increase the size of the marker symbol so that output mask features will simulate a large mask around a small feature.