Here’s a pho to

The photo didn't upload. You have to wait until it becomes full color before submitting it. Hopefully, it is not a close-up of the grass, but an overall shot of the area that shows us what the conditions are like, including the tree or object that produces the shade.

It is the shade. I lost tons of St Augustine last fall where it was thin because of heave shade and way too much rain last fall. The grass is just not vigorous enough in shade to overcome all of the rain. This was grass that I had kept going for many years in mostly shade. It just could not survive the heavy rain.

Shade is not the issue.

Does water pool in this area?

What is your watering schedule? How often and for how long do you water?

The solution is to apply cornmeal to the entire lawn at a rate of 15-20 pounds per 1,000 square feet. Call your local feed stores to check price and availability. It should be around $10 or less for 50 pounds. If they quote you $30 or so, they are talking about corn gluten meal. That's not what you want. You want ordinary whole ground corn.

Give the corn meal 3 weeks to work. But if you are watering too much, then the problem will return.

Water doesn’t pool , but soil is still moist 10 days after last rain. I haven’t watered in weeks

"Hopefully, it [photo] is not a close-up of the grass." A photo would show the tree canopy and its shadow, so we could see if it is the problem. Here, you're showing just the bottom of the trunk, which leaves us disagreeing about the tree's effect on the grass.