I found that too when I started if you use the same size rope for the hitch as your climb line, my climber friend does this and works for him, you really have to work the blakes tight I found, but I'm much heavier then he is. So I went to a Bee Line prusik cord, and it holds amazing now, if you have one of these don't do the blakes too tight cause it holds too well and you'll find yourself stuck in the tree!! Ask me how I know. After I used it a few times it loosened up and now it runs great you can repel down fast then let go of it and it stops me instantly, but have to be careful doing this with a blakes cause it gets real hot.
Got it from Baileys. They have 3 different lengths, 24", 30", 36".
Bailey's - Yale 30" Bee-line & Prusik Cord