^ Just what Dutch Pimp said.

Another possible contributing factor making the high less enjoyable is CBN levels:
Quote Originally Posted by http://www.a1b2c3.com/drugs/mj028.htm

CBN (Cannabinol) is produced as THC ages and breaks down, this process is known as oxidization. High levels of CBN tend to make the user feel messed up rather than high.

CBN levels can be kept to a minimum by storing cannabis products in a dark, cool, airtight environment. Marijuana should be dry prior to storage, and may have to be dried again after being stored somewhere that is humid.