If it's Kentucky bluegrass, spotty coverage is sort of the norm. As the grass develops next year, it will fill it in. However don't expect full coverage for a good long while. If it's rye or fescue, more seed is the only way to get more coverage. The thin stalks will develop into bushier plants as tillers develop, but they don't spread. Is the soil still warm? Here in Virginia we're having cool mornings and warm afternoons and no frost forecast for the next two weeks. I'm putting a little more seed (fescue) into particular spots today and pushing it into the ground with my foot. Can you remove the straw in the areas you want to put more seed? Maybe mow it?

Where do you live? The zone isn't that helpful. SoCal is full of soil types as well as micro climates from mountains and wind patterns. What is your watering schedule? How often and for how long do you water? Why did you want clover? There is no mix of fescue and rye that will work in SoCal unless you live west of Interstate 5 on the coast. If you don't have morning fog, a 2pm sea breeze, and high temps in the 70s all summer long, then you have no business trying to grow a cool season grass. Most of SoCal should have either St Augustine or bermuda. If you are pretty successful at caring for a lawn, then you could try zoysia. Another excellent choice for the really arid areas is UC Verde buffalo grass. Any of these grasses need water only once a week in the hottest heat of summer and more like once a month the rest of the year.

Let's start with a lawn calendar. You've been off calendar so far. March/April - at the first sign of forsythia blooming in your neighborhood, use a preemergent herbicide. Mid April - spot spray broadleaf weeds (clover, dandelion, spurge, etc.) with something like Weed-b-Gon or Weed-b-Gon Chickweed, Clover, and Oxalis Killer Late May - fertilize for the first time with a plain fertilizer, not weed n feed. Early September - fertilize for the second time, no weed n feed. About this same time you can overseed if you need to. Mid September - spot spray broadleaf weeds again Early October - evaluate your overseeding efforts to see if you need more seed. Late November - when the grass stops growing but before it changes color or you get a hard freeze, apply a high N fast release fertilizer like uncoated urea. Do not get tempted (by TV commercials) to seed in the spring, and don't get tempted to fertilize early. The late fall fertilizer will awaken the grass early in the spring and cause it to grow fast. If you miss the fall application of chemical fertilizer, you can apply any organic fertilizer at any time. Organics have a different way of fertilizing, so all the cautions go away. Don't bring in any new topsoil unless you have low spots you need to fill. Topsoil and sand will always change your drainage and often for the worse. The last thing you want to do is build a dam that holds water up against your home's foundation. Besides grass seed sprouts at the surface, not buried under a light app of topsoil. It will sprout under light soil, but if you don't need it, don't do it. For this upcoming overseed, I would get a tri-mix of Kentucky bluegrass, rye, and fescue. If that is what the Scott's product has then fine. The KBG and rye will do well in the full sun while the fescue excels in the shade. Overseed the entire yard to get the colors to blend in. The newer seed could be deeper green than your current turf. Mow the lawn at the mower's lowest setting, remove the loose grass, apply the seed, and press it down with a rented, water-fillable roller. Watering the tri-mix is a little tricky because the three seed types germinate at different times. First of all you water new seed to keep it moist, not soggy, all the time. Usually that means 10 minutes three times a day for 1, 2, and/or 3 weeks as follows. Rye comes first at one week. Fescue comes at 2 weeks and KBG takes 3 weeks. So if you watered for only one week, all you would get is rye grass with just a small bit of early germinating fescue. There would be no KBG. The KBG has the advantage of spreading to fill thin spots, but it has the disadvantage of sometimes turning brown in a harsh winter. Since you're new to this, here's more on watering mature grass after the seed is up and growing. Watering: Deep and infrequent is the mantra for watering. This is for all turf grass all over the place. Deep means 1 inch all at one time. Put some cat food or tuna cans around the yard, and time how long it takes your sprinkler(s) to fill all the cans. Memorize that time. That will be the time you water from now on. My hose, sprinkler and water pressure takes 8 full hours to fill the cans. Your time will likely be less. I like gentle watering. As for watering frequency, that depends on the daytime air temperature. With temps in the 90s, deep water once per week. With temps in the 80s, deep water once every 2 weeks. With temps in the 70s, deep water once every 3 weeks. With temps below 70, deep water once a month. Note that you have to keep up with quickly changing temps in the spring and fall. This deep and infrequent schedule works in Phoenix and in Vermont, so it should work for you. The reason for deep and infrequent is to grow deeper, more drought resistant roots and to allow the soil to dry completely at the surface for several days before watering again. If it rains, reset your calendar to account for the rainfall. Mow at your mower's highest setting for most of the year.

Late summer is after the heat breaks for YOU in your area. If you live on Hudson's Bay, you could probably do it now. If you live south of Detroit, you should wait until the end of August. Usually the reason for seeding in late summer is that there is ample time before the first frost causes dormancy. It also gives you time to evaluate the coverage and add more seed in a few weeks if necessary. Clover should not need to be reseeded, because it reseeds prolifically. As for covering with 1/4-inch of topsoil, instead use 1/4-inch of compost. Adding topsoil when you have good drainage already is going to mess up your drainage possibly resulting in water in the basement. Note that as clover reseeds itself (prolifically) it does not cover itself with 1/4-inch of anything, and it grows just fine.