Check the situation - is the area under trees? affected by the roots of a large nearby shrub? Has something been spilt on the groung there? Or dripping off a building, fence or plant?
Dig up & check the soil for root eating pests. Check for buried rubble/foundations that will affect roots or water supply in a drought?
Check the soil for acidity, & mineral balance. Too acidic means the grass just won't thrive & other soil nutrients may be unavailable. Nitrogen, Potassium & Phosphorous balance may be affected. Then correct any problems discovered & replant. good luck.