I believe that your projection screen is the culprit. If you are bound to the type of screen for so long, you'll have to create a special pen for it. A backwards facing pen, or a pen with LED in the front, but tilted at an angle so that your wiimote sees it directly would do the trick. Much cheaper than buying a new projection screen.
I suggest you change the sensitivity settings inside smoothboard maybe the IR is being to dispersed.
Fastolfe Just worth trying, you never know. By the way what IR-Pen are you using (IR-Led & Battery) as it may even be an IR-pen problem.