MouseHit() does not work sometimes

Any clues why MouseHit() isn't reliable on my pc? It's a dual core machine and very fast, however MouseHit often 'misses' or fails to pick up my clicks. I find I need to call it a LOT in order to get some kind of response from it, as in more than once per loop!

I don't have this problem with keyboard input nor do I have a problem with any other games or applications.