Why don't my wiper blades clean my windshield well?

First of all take a look at your wiper blade and see is the rubber still flexible or is it really stiff? If it's really stiff, some fresh blades are probably in order, especially here in Mesa, Arizona we tend to see them dry rot. Secondly if you run your blade about halfway up the windshield, check to see if it's making contact all the way. If not, there may be some issues with the arm itself that holds the rubber.