This Allen St. Augusine lawn may have been damaged by using too many chemicals. It is always a tough balance with St. Augustine to apply enough weed control to prevent or kill weeds but not so much that lawn becomes damaged. Since it is a tropical plant it is already stressed from too much heat and cold and when chemicals are blanket applied such as weed and feed products it can put the lawn over the edge. Notice there is almost a straight line between the healthy lawn and the stressed one? There is plenty of sun here and with correct watering this Allen lawn will come back but it will take longer than other lawns might.