Keyboards work funny when it comes to simultaneous keys. It's unreasonable for them to be able to send information about every single key telling which ones are being pressed, so they have limits.
When I was growing up, all the keyboards I used had a three key limit, period (though that might have been a quirk of Windows Me, not the keyboards). You couldn't hold more than three non-modifier (as in, not Shift, Ctrl, or Alt) keys at any given time, or it would beep at you.
These days it's different. On all the keyboards I have right now, it is not key-count but rather certain combinations of keys don't work, so the limit varies depending on the keys you try to press in combination.
Remap the jump key to something else, and hopefully it should work.