Effect of terrain to the movement of gameobjects

I’m building a car race game using unity. For that I’ve added sketchup 3d models of vehicles and tried to move them using nav mesh agent component. Some models move perfectly while others float, sink, rotate and reach to the goal in reverse. I changed, repositioned colliders, added force, freezed the rotation and position (simply did all the solutions available on the internet) but no change. So I thought this happens because the models are sketchup models. Therefore I addaed a simple cube but it shows the same behavior. Does it mean this abnormal movement is caused by the terrain?? How can I fixed this problem and get a normal nav agent behavior?

From your description it’s not entirely clear what is the issue. When you say that some models move perfectly while others sink - are those distinctions consistent? The same models always misbehave?
Do different models have different scripts attached to them, or the same scripts with different parameters?

Do they have rigid bodies attached to them? differently shaped colliders?

I would start by trying to replace the models that fail with those that work, and check whether the issue is with the model itself, or with some other parameter. If the model is the issue, try tweaking it until you understand what causes the issue.