As others have said, you have to increase the development time the more you dilute the developer.
Originally Posted by ezwriter
But developer is diluted for practical reasons, particularly to give you realistic development times. Stock solutions may only take two or three minutes to develop a film, which means everything has to be timed to perfection and agitated to perfection, small discrepancies in temperature, and time making big differences to the negs. So diluted the developer is more gentle, allowing small differences in agitation, temperature and time, and making more predictable processing possible. Developer is also diluted to control contrast, by which the developer becomes exhausted in the highlights before it does in the shadows, so giving better shadow detail and less chance of blown highlight detail.