The adjustment feature in the prusic knot comes in the fact that you can slide it up and down the length of rope which you' ve double back around brought through the loop ... the " tag" end of the rope with the loop in it.

I still can' t figure out why that rope with the loop in it would slide down... did you get a really soft rope (the one with the loop in it)? I mean, mine' s so soft, it' s almost got fuzzy threads from the cotton sticking out of it. There' s no way mine' s going anywhere once I cinch it up.