It's probably failing because of the Ribs not completly intersecting the side of the base at the mirror side. Maybe you should try to build the boss without merging the bodies and then try to mirror the body instead of features. If that works then you can use the combine command to join the features later.
I think I got it. I had used a regular sketched point on the surface as the reference. I deleted it and did a 3D sketch on plane (same surface) and the mirror worked.