If you know that the glass is clean then try cleaning the blades.
To clean the blades, use concentrated (not pre mixed) wiper fluid or rubbing alcohol. Dampen a cloth or paper towel with the cleaner or alcohol, and wipe the rubbe part of the blades. You will be amazed at the black that comes off. Repeat until no more black comes off the blades. The black is a combination of dead rubber and road grime.
As a sort of ongoing maintenance, clean the blades with the same stuff that you clean the windshield with each time you clean the windshield. The blades will last a lot longer and they will clean more consistantly.