I'm by no means an expert, but I don't believe play sand really adds any benefit to the soil. I would suggest using a combination of Texas Green Sand and Lava Sand instead.
Go ahead and rule out the common culprits... grubs, root fungus, poor watering habits, etc. There's plenty on this forum about that. You might want to consider working in a 1/4 inch of high quality compost top dressing, but that might be jumping the gun as most here prefer patience, it's still early in the St Augustine season.
And let me be the first to point out, "if you had been on the organic problem all along you wouldn't have this problem"
Ugh... that line still kills me! 