"certain strains of top quality marijuana may be more potent than certain types of low quality hashish. As hashish is exposed to air, light, and heat it loses potency. The longer hash is stored, the more potency it loses. Most hash from Asia and the Middle East takes months or years to reach its destination from the time it was produced."
http://www.a1b2c3.com/drugs/hash013.htm

Damn, I had to correct that. They had both spellings of "loses" with an extra "o".