I think I have a problem with expanding the food warehouse. I am constantly expanding but the amount of food remains the same, it does not increase.
What building do you upgrading ?
If the Food Storage or Advanced Food Storage, then the maximum food storage should increased.
If you upgrade a Farm or Watchtower, then it is only increasing the Temporary Food Storage what the Farm/Watchtower can hold before it is start producing more food.
Like @PlayForFun asked, what level(s) are your four food warehouses? The incremental gains in food storage for upgrading an advanced food warehouse are relatively small per level.
This is the only building which increasing your food storage once you upgrading it. (And of course the advanced version of it)