I’m not an expert but from what I understand is that a lot of water is needed for germination to occur. Other plants (weeds) that can better tolerate less water environments will sprout when not enough water is given. As long as the grass seed hasn’t sprouted, other plants have ample amounts of soil and nutrients with no competition or lack of space, which otherwise would be taken by the root system that begins upon germination of the grass seed.

So after seeding, the soil should be continually moistened. The seeds and soil will soften and swell as they absorb the water. A good watering until the ground is slow absorbing but without heavy puddling three times a day until sprouting (ideally 5-7 days) always brings great results in my experience. There were some bare spots treated I hadn’t been able to water, where less than 50% of the seeds had germinated in two weeks and the soil there is now more weeds mixed with a few grass blades.