The wiper arm is out of alignment and the blade needs to be at an exactly 90degree angle to the windshield. This may be caused by trying to run frozen wipers.
The wiper arm load spring is not working correctly - this usually causes the middle section of a beam blade to not touch the windshield or the edges of a traditional blade not to touch.
Improper sized wipers - they may be hitting the top or sides of the windshield, each other or not clearing enough windshield to safely drive.
Wiper frames have not been replaced and they are corroded thus not allowing for an even wipe as the blades travel across a curved windshield
Blades installed upside down. Yes some blades have to be installed right side up or they will cause wind drag and lift at high road speeds.
Not removing the rubber edge protector off the blades at install
Blades not installed properly and thus they may flop/wiggle loosely while running causing an inconsistent wipes.
Worn out wipers. Sun, heat, cold, time, debris, and use will cause blades to wear/dry out causing performance issues.
Cheap discount wipers/old wipers may not flex properly to todays modern curved windshields or the wiping surface deteriorates or cheap materials will cause an unsafe environment when you to depend on it most.