I didnt mean ONLY retracting. I mean a quick jab striking me in the jaw and then retracting.
Most jabs are probing. If you have been struck on the jaw, you obviously did not evade or cover and you probably are not in position to counter immediately. If the jab is more probing then you probably will just cover or evade.
However, just because a jab is being retracted, and as such doesn't have great momentum, so what? Where is it written that aikido needs always to redirect energy? If I can enter, I will continue and take out my opponent regardless of his action or lack of action. It is about taking your opponent's centre.