Then your problem must be with the PIR detector and how fast it activates.
Maybe try another brand of PIR detector. When feeding the signal into a computer I would prefer a PIR detector with as short output signal as possible and let the computer stretch the pulse.
The timer value defines of long it stay on after the PIR turns off, it is used to keep the output permanently on if the PIR output flickers off/on (In retrigable mode it is not supposed to do that very much).