Answer: It sounds as though the soil is compacted. A lawn can become bumpy due to the freeze/thaw cycles of winter, or it can become lumpy if moles or voles have been busy tunnelling around. I think I would rent a core aerator and aerate the lawn. A core aerator will remove one inch by three inch plugs from the lawn. Leave the plugs on the lawn and they will dissolve in rain or water from the sprinklers. After aerating spread a thin layer of sand or compost over the area and water it in well. The sand or compost plus the soil from the plugs will work their way down into the holes left by the plugs. You can then overseed your entire lawn or just let the grass grow back all on its own. You'll see a big improvement after aerating your lawn.
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