Perhaps a difference in the size of the hits? I find that while you can get high on moderately sized hits, you get exponentially more higher by taking those hits so big that it leaves you reeling like you just took a punch to the face. It's hits like these which give me what I call the "money shot", the hit which forcefully propels you into your high. With me, the money shot doesn't happen every sesh - if I only take moderately sized hits for some reason, the high just creeps up on me.