Answer: Based on your description I am not certain what is happening to your lawn. However, Bermuda grass needs full sun all day long so if it is planted in a shady area it just will not thrive.
Finegrass Bermuda is usually started by sprigs or by sod, it is not seeded, while common Bermuda can be seeded. Bermuda spreads naturally to fill in and become thick so if it is healthy you should not need to add more.
Bermuda has special care needs. It must be fertilize correctly (lightly every six weeks) and mowed correctly (at a height of one inch and frequently, never removing more than one third its height at a time) and watered correctly to stay healthy. You may find the following information helpful in working out your maintenance plan. You may need to cut and paste the complete url into your browser to make it work correctly.
If you have not already done so, I would strongly suggest you run some basic soil tests to check the fertility and pH of the soil in your yard. Your local county extension should be able to help you with the soil testing and interpreting the results. They may also have suggestions on how to improve your lawn. And, they should be able to diagnose the cause of the brown spots.
Browning can have many causes ranging from poorly rooted sod to pest or disease to uneven watering to accidental overfertilizing or chemical exposure to dog urine and so on. It is very difficult to diagnose long distance. If a chemical control is needed the county extension will have the most up to date information on what to use and how/when to use it for best results.
I hope this helps!
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