This is called Zebra stripping and can be caused from 4 areas of concern. Flip cover over and check fan Pattern to make sure you are getting full spray
Out of each nozzle.
Sometimes when you connect garden hose to pressure washer you can pick up Debris and will make its way to the nozzle and will cause this issue from partial fan spray coming out.
Remove the nozzle with 9/16 socket and use air compressor or tooth pick And clear orifice of Tips.