On the surface, you would presume that agile development and user centred design techniques are a good match. They can be, but not necessarily. As Jared points out, the agile sprint process doesn't necessarily encourage a return to work already done to appraise and improve it.
When you're caught in the situation where you can't look back and adapt development that's already happened, from a user experience perspective you might as well be developing using the waterfall method.
One of the things I enjoy doing when I'm talking to a group of Agile practitioners is to ask, 'How many of you get to iterate a particular piece of functionality through three sprints until you get it right?' No one raises their hands. 'Two sprints?' No one raises their hands. 'One sprint?' Now I get some hands. The idea that you're going to go back and re-work functionality is hard—organizationally hard, emotionally hard, and practically hard—because then your schedule slips. So yeah, we like the idea of iteration. The actuality of iteration, not so much.So the article goes through some of the places where agile processes can catch out a team trying to enhance the usability of the product that they're working on, but also suggests some techniques to address them.
Cost Effective Approaches to Iteration in Agile UX - article by Jared Spool for uie.com