Yes, Alex is right. Imagine, that your game runs very slowly and combo sends keys at standard rate, game accepts one key, but may skip second.
I am doing now something similar in Wizardry 7. This game has stupid system for showing minimap, however can be fixed with deadly strike. But here is problem, that I must wait for showing character screen who holds map kit. However character screen appears in range 300ms-2seconds. I don’t know why.