Could be drying out faster than it is rooting. Give it a pull to see if it comes up. Check your sprinkler coverage and also the underlying soil for compacted spots or spots where the sod isn't fully contacting the ground. Does any of that help determine the issue?

If some spots are taking root, give it a bit more time before pulling it up. St. Augustine can sometimes stall out depending on soil compaction.

Appropriate watering for new sod is 3x daily for 5 to 20 minutes each time continuing for 2-3 weeks before backing off on the watering frequency. Never let the soil get soggy/mushy. The idea is to keep the soil that came with the sod moist as well as the underlying soil/sand. If you have a coarse sandy soil, then you should water more than 3x per day. You also might try covering the grass with burlap or other shade cloth. Is it a big area? If you can save any of it, then it will eventually spread to fill the rest.