Not sure what is really happening with your situation, but when I use the "Left Shift" button, it is so I can place something like the floor piece in the middle of the wall instead of the actual snap-to location. Without use the Left Shift button, you have to get the cursor "close" to the snap-to part of the object, in your case, this would be the edge of the floor piece or the end of a beam. This can be a pain sometimes depending on were you are standing and your view angle. At least that has been my experience.