We have two big canopy oaks on each side of it. Where the grass grows under the canopy isn't the big problem. It's outside of where the grass sort of does not grown.

A number of sandy area, where grass has receded, over the last two-three years. Not sure why.

What would the right way to figure out what's going on be? I'd like to really get in the know, so I can figure out what needs to be done.

Yes but we need to know where you live and what kind of grass you have. And what kind of soil if it isn't completely obvious that you have sand. Like tiemco did, I'm going to assume you have St Aug, sand, and you're not watering or fertilizing at all. Otherwise you would have dense turf in the sun and slightly less dense turf in the shade.

St Augustine care guide:
Water deeply when you water but don't water until the grass tells you. When the first blade of grass changes color or looks wilted, water with a full inch of water. Measure how long that takes your watering system to apply a full inch by setting out cat food (or tuna) cans. The time it takes to fill those cans is your 1-inch time. For some systems it is 15 minutes. For my system it takes a full 8 hours. Note that the cost for 1 inch of water is the same whether it takes 15 minutes to apply or 8 hours. One big advantage to a slow system is you never get any runoff. Note also that water applied in the shade will seem to last longer than in the sun. You will need to favor the sunny parts of the yard with your sprinkler. General watering frequency is monthly in the cool months and weekly in the heat of summer. Your sand, shade, wind, temp, and humidity conditions will dictate the ideal times in the summer.

Mulch mow St Augustine at your mower's highest setting. Short St Aug will dry out quickly. By the way, St Aug does not go dormant from drought, it goes dead. Tall St Aug grows deeper roots and holds water well and helps shade the soil to keep it cooler in the summer heat. You NEVER need to mow St Augustine any lower than the highest setting on your mower. In fact I'm experimenting with my lawn by not mowing it at all. In the shady parts the grass is knee high. In the sun it is 4 inches high. I need to favor my sunny areas, too. But don't ever let anyone do you a favor by mowing your lawn short. It can take several weeks to grow out back long again.

Fertilizing
Since you really need organic matter in your sand, the only fertilizer you should be considering is organic. Probably the easiest organic fertilizer to find around the country is rabbit food (alfalfa pellets). Apply alfalfa at 20 pounds per 1,000 square feet every month (or every week if you can afford the alfalfa and the time to apply). Just toss it out on the lawn and moisten it. You don't have to water it in, just moisten it enough so that the pellets swell up. A very light mist is enough. That keeps the birds from coming in and stealing your fertilizer. If you are not familiar with modern organic lawn care, here is a picture of a zoysia lawn that had a handful of alfalfa applied in early May. The photo was taken in late may.

Note the color and density differences. Alfalfa makes a very good, and inexpensive, fertilizer. You can find it at feed stores everywhere. Cost is about $12 for a plain brown bag, 50 pounds.

If all the assumptions are correct, then the above is the solution. You could easily have the best looking lawn in the neighborhood by May. Start now in the Panhandle.

Yes, Were in Pensacola, and my grass is St Augustine.

Lots of good info. Appreciate.

Sand is great for St Augustine. I've seen it get up to 30 inches high growing wild in the dunes in Texas. When we bought our house the soil had washed away after decades of no grass. We started bringing in topsoil to rebuild the grade. I ended up buying sand and wish now I had used only sand. The places where the sand is the grass is noticeably healthier with less input.