I would go with one of the rods being out of place on one end or the other. If it moves a little after you pull the handle, but not before, something is moving in there. I know when I was putting mine back together, I found that if the striker plates were mis adjusted, it would bind up and not open, even though the latch mechanism in the tailgate was working fine.
How much does it move when you have the handle pulled all the way out? If its more than a 1/4 inch or so, it might just be binding up on the striker plates.
If its not moving very much at all, I'm with the other guys in saying take the handle off and pull the rods together manually.
Good luck!