Try digging up the topsoil in the area and putting in new soil, then reseeding, or resodding.

If previous companies have had no success, I would recommend a soil test. The soil test will revile any toxins or lack of nutrients. If the "brown spot area" is under shade it may pose a problem as well.

Could be "brown patch" caused by a fungus which can be eliminated with treatment. Your local County Extenstion office can most likely diagnose it for you if you take a sample of the turf in. Could also be "root rot" if there it occurs in a low spot where water stands and does not readily drain.

Put in zoysia plugs. Zoysia is a hot weather grass that does great in summer. I been plugging my yard and it is really starting to look great. Start with the brown area.