Welcome to the forum.
I live in south Louisiana. St. Augustine is the grass of choice. There are many things that can cause the conditions that you describe.
I can not even begin to answer you questions without more information.
What is your location - specifically? What are your soil conditions? How much rainfall do you get? Do you have a sprinkler system?
There are many variables that can have a negative effect on your St. Augustine.
Elizabeth - or Your Majesty
Living and growing in Lafayette, La.
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant. ~Author Unknown