I'm having problems using the UV mapping in blender 2.67. Specifically, I am trying to unwrap a really simple mesh, but the UV map is still giving me a net where some of the boxes are completely out of proportion to the others, which causes the texture to stretch horribly. Here is a picture to illustrate:
The mesh only has those three faces you can see in the picture, and the single seam on the front corner. It's a trivially simple mesh, so I really don't understand why the two side faces are being stretched in the UV map to be the same size as the top face. I've tried clicking the average islands scale button as suggested by some of the answers to other questions but that has no effect. It's frustrating because it seems so simple.
Hopefully someone can show me the presumably simple mistake I am making!
Edit: Just to make this clearer, I have already added a seam as the first answer suggests i do. Hopefully this picture will make that clearer, the yellow arrow is pointing to the edge i seamed. The mesh is only three faces so should only need this single seam. The image shows the mesh from infront and behind/underneath so it should make more sense.
Double Edit: One more picture for clarity on the seam issue. The mesh is unfolding correctly, it is just the scale of the unfolded map that is incorrect (in relation to one another)