I'm unable to trim the angled pieces in this weldment model.  All of the perpendicular pieces worked fine, and I've done this very thing in other weldments. 

The Trim/Extend operation shows the correct preview, but once the green check is selected, there is no change in the model!

I'm simply trying to trim the 2" x 3" gussets up to the face of the 3" x 3" square tube.