If you are trying to control crabgrass, the preventive barrier must be applied before annual grassy weeds germinate; at soil temperatures of at least 55 degrees. After that time, weeds will germinate and applying a preventer will not work. If you miss apply, leaving missed strips in the lawn, you will find weeds in each and every missed strip. Be sure to overlap for complete coverage.
If your lawn has broadleaf [dicot] weeds, like dandelion and thistle, weeds must be treated when actively growing. Using a dry product, the material must be applied to moist foliage. That can mean watering before making application or rising early enough in the morning to apply product on a dewy lawn. Liquid controls use water to ensure weed coverage and contact with the blade, where the weed control is taken in and translocated throughout the plant for complete control.
Finally, some weeds cannot be selectively controlled; that is, killed off while not harming the desirable turf grass. Be sure the weeds you are trying to control, are on the list of weeds for which your weed control product is registered. See the product label for specific weeds.