Hello, My XC540 will not do a sheet cut. I have changed the blade, pressed the spring back to see if that was jammed. Cutter goes all the way to left and comes back like it is cutting but for some reason isnt. How is blade supposed to stay in down position for cutting? Wonder if i am missing a part or something. the silver looking thing that sticks out, pushes the blade down into the cutting posistion but seems like it wont stay there.