I'm always looking for good grass.What kind you got?
Grass needs to be good and wet when you are applying it so the stuff will stick to the weed surface. If the dew isn't heavy then mist down you lawn with a hose sprayer before spreading. When I do it that way I do a section at a time so I can get it spread before it dries off then move to another section.
Use a Surfactant
That might help. I mix a little in when I spray my Weed Stop.Stuff dries quick in the Texas summer when it is already 90f by 8am.