Problem with multiple button presses on keyboard

I've been playing Stepmania for a couple weeks now. Getting the hang of it but I've noticed a few hits I shouldn't miss. I've come to find that I can't make certain inputs while playing. I've read that some keyboards just can't handle more than two buttons at a time, but I find I can press all three keys of any combination except Left, Right, and Up. Any ideas? Left Right Down works, So does Left Up Down and Right Up Down...But for some reason not Left Right Up....and it doesn't seem to matter if I re-assign the keys to different buttons, it still happens.

Use the input test screen to find four keys that work together. It's under Options, then Input Options, then Test Input on the main menu. It'll show you every key that stepmania currently detects as pressed, like this.

You'll have to try lots of different combinations before you find four that work together. This is a problem inherent in the way most keyboards are designed an manufactured, you can see it happen in any program that shows you the keys that are currently pressed. Some keyboards have "n key rollover", which means the manufacturer put a lot of extra design work in to allow any combination of keys to work together.