Answer: You didn't describe the patches, so I'm going out on a limb here - There are many reasons why dead patches appear in lawns. The most common causes are:
Gasoline spills or leaks from the mower.
Uneven application of weed killers.
Uneven application of fertilizers.
Inadequate preparation before sowing grass seed (may mean there is buried debris or an uneven surface).
Badly laid sod or uneven surface.
Uneven surface is 'scalped' while mowing.
Dog urine - douse copiously with water as soon as possible.
Waterlogged roots (water deeply but infrequently - one inch of water per week is usually adequate. To judge how much water your sprinkler is supplying, place empty tuna or cat food cans on the lawn and water as usual. After 15 minutes turn off the water and check the level of water in the cans. This test will indicate how long you must run your sprinkler in order to supply one-inch of water to your lawn. Adjust how long you keep the sprinkler on by how much or little water collects in the cans).
Hope this provides some insight as to what the problems(s) may be!
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