It has to be either (a) the focusing screen is defective or isn't properly installed, or (b) the mirror is out of adjustment. It can't be the fault of the viewfinder since it only re-orients the image from the focusing screen. With a basic (i.e., non-magnifying) waist level finder, you are essentially removing the prism viewfinder and focusing directly off the focusing screen. The waist level finder is nothing more than a hood to block stray light. Check to see if your mirror is all the way down and seated.