I've made a simle part, the sketch of a rectangle and circle.  Then extruded that.  The sketch is intended to be 1/4 of a symmetrical part (symmetrical horizontally and vertically).  So I mirrored it horizonatlly, no problems.  Then when I mirror that mirror to finish the part, I choose the mirror to mirror, choose the plane of the first mirror to use as the face to mirror about, and SolidWorks shows the intended new mirror correctly but when clicking on the checkmark I get rebuild error "Could not find face or plane".     I've tried a bunch of times, even made a new plane to use as the mirroring plane for the second mirror to no avail, same message.  What am I doing wrong?