Uproot one of them and take a look at the root system. Also dig down
and see how far the soil is actually moist. And consider the temperature...
high temps can make a plant just "stall" (it's called "heat shock" or "heat
stress").

What's the pH of the soil? How salinized might it be?

What's the temperature, day and night?


Are you sure there's not been any contamination with one of the herbicides?

Oh, and 6 hours of sun is just barely adequate for most tomato cultivars.

You might also take the dug up tomato and pot it in a 5 gallon or more
pot with commercial potting soil, and see if that improves its growth.

Kay