Do you have any examples of your works where this happens?
Usually the reason why models look like they're 'floating' or 'just there' is because you're not putting any real interaction with a background in the drawing.
When a character sits on a surface, their butt and thighs squish because the weight presses down on the surface, causing the mass of the buttocks to spread. If you don't convey the squish onto the surface properly, it will look like they're not sitting on it/not putting their weight down.
Then there's also the perspective and where in the drawing you put the focus on. If your background doesn't look realistic or proportionate to your subject, your subject will not look like they're part of the environment. They'll always look out of place!
My suggestion is to practice with models that interact with the environment. Try to draw out the background first, then gently nudge the character in there. If you draw the character first and then the background, you increase the risk of the subject not being integrated properly.
I hope this helps! If you have any examples to show, we'll be able to give you more specific pointers. :)