It may be a case of keyboard rollover: Key rollover - Wikipedia
To prove or disprove it, make a simple script that displays the different keys that you need held down when they are held down, see if that simple script can show all the keys at once that you intend to hold down simultaneously.
If it can, then you may have a bug in your as-yet-unposted code. If the keys can’t all be held at once, that’s your keyboard’s limitation.