A lot of people get the huge libido boost initially but as other hormones start to fluctuate, they lose that libido in a hurry. Libido is a complicated function and is affected by a number of factors other than test and free test levels.
There are a number of factors including Testosterone, Estrogen, Progesterone, prolactin and serotonin. One possible explanation could also have to do with the pituitary. Many anti depressants, in the class of serotonin reuptake inhibitors also lower libido. So there could be some relationship with pituitary shutdown due to AAS use that could cause this.
many possible scenarios