When retrieving the rope following an Munter hitch rappel, care must be taken that any twisting in the ropes is removed before pulling them down through the anchor. If you don't do this, there can be so much friction from the ropes crossing each other that it can be impossible to pull down your rope.
Here’s a simple way to do this.
Before you leave the stance at the start of a rappel, clip a carabiner onto one of the rope strands, and let it slide down and sit on top of your Munter hitch. As you rappel, the ropes will twist slightly, stopping this free running carabiner from staying with you. When you reach safe ground, unclip the Munter hitch and start to untwist the ropes by hand. When the free running carabiner drops down to you, you know that there are no more twists in the rope, and can start to pull it down.
From the book “The Complete Guide to Climbing and Mountaineering”, by Pete Hill