I can't grow a full beard either. Pisses me off.
They say thinking about sex makes produces testosterone and testosterone is what makes beards grow.
Facial hair is largely governed by production of testosterone, which is known to be secreted by the testicles in greater quantities during and in anticipation of intercourse. Scientists have also found that increased beard growth was associated with tension, mental fatigue, alcohol and increased shaving. On the other hand, it is apparently curtailed by heavy exercise and high temperatures.