How come Nyquil doesn't put me to sleep?

So i have been really sick for the last 3 days with a cold and during those nights before bed i've been taking the recommended dosage of nyquil. It doesn't even make me drowsy but my sister says that when ever she takes it she knocks out. My cold has been keeping me up and I just want to have a full nights rest. Why doesn't Nyquil put me to sleep? Should I take a little more than the recommended dosage?