When you look at the foundation pieces, are they blue or green? Blue means they are attached to the ground, If they are green you may need to place a beam, destroy floor, place beam under beam; personally I use build cam to make this easy.

Do you use any mods? Popular mod Valheim Plus if the "free rotation" part is enabled the hoe will not level to your feet, it doesnt work properly. Destroyed tons of houses until I disabled this part of v+ (it leaves just enough gap under the floor to be not supported by ground)