Multi-key keyboard input (Flash MX)

Hi. I'm currently developing an application in Flash MX 2004 Pro and having a problem when using multi-key keyboard input. I don't think it's an actionscript issue, as the code is working just fine with some key combinations - just not others! The keys i'm using are S D F J K L and i'm getting the desired effects with a few or ALL of the keys pressed, but not with certain combinations of four or five of the keys pressed at once. Very odd!

Obviously keyboards weren't designed to have several character keys pressed in unison but it does seem to work fine with some key combos and not others (for all our sanity i won't reproduce the problem combos here). Essentially, is there a keyboard circuitry issue that makes some key combos unrecognisable by computers (thus a hardware issue) or is it an issue with my AS or the Flash Player itself? I need to know if i'm wasting my time trying to solve this problem.

Any insight would be much appreciated! Cheers.