Do you have Crab Grass?
Once you have Crab Grass you have to treat it with a post emergent spray. You may have break through for a number of reasons. Maybe you didn’t get a chance to get the Pre emergent down, or you did get it down and the rain has diluted it. It could also be because of thin turf or heat stress along driveways this also allows crab grass to move in. So proper watering is key during stressful times. Another helpful thing would be to aerate and over seed with a better more drought tolerant grass. The Aeration will help thicken and strengthen the existing turf and open it up for the new grass seed.