Why is my tomato soup not thick?

The soup needed to be simmered. Any type of thickener need time to bind with the liquid.
Next time use dried mashed potato flakes.

you need to boil anything that has had thickener added so it comes up to its full thickness.

Starch products need to bind to fat for them to thicken. Bring your soup to a low boil and add butter, a pat at a time, until desired thickness is reached. And yes, you can use an oil instead, depends on the flavor you want.

add some corn starch.

You said you added corn FLOUR? Generally to thicken stuff you add roux (heated and mixed butter/animal fat and flour) or a slurry (cornSTARCH and water). I'm not sure if cornflour works the same as corn starch, but i know that cornstarch would definitely thicken up your soup. If not, then you should've added some butter to the soup.

Also boiling down and reducing the soup would help to thicken the soup a little bit, depending how much water was in the soup, but you also risk evaporating a decent amount of your yield

Could be the quality of the soup

add heavy whipping cream.