Hi;
This is a problem usually associated with two major things. First the Chassis Plate or frame may be warped, Or and here's the kicker! Maybe you accidentally put something on upside down.
What I do with model cars is this. When I open the box I first extract the frame or chassis. I sit it on the worktable and lean down and check to see if it is sitting level. If not a few twists to see if this fixes the problem. Gently though!
Now if I don't have that to worry about that, I make sure I fully am aware of all the parts and their correct position on the vehicle. Sometimes, ( Don't laugh) the car is fine. It is the rear or front suspension that is the culprit. I used to have that problem with any model car that had separate leaf springs in the rear!
Why? Well, like rear cars, Plastic leaf springs have an arch to them. Being plastic they will flatten out over time. So when you put them on, You have a car that looks like it's lifting it's leg on the furniture.! So check them out and make gentle adjustments where needed. If the shocks are mounted properly they will support the model and it's plastic springs just fine.
Now if the vehicle has Independent suspension front and rear. It could be this. Tamiya and Hasegawa Many times mold the Bottom "A" arm to the chassis Plate, Warped plate Floppy model! So check this out and correct it. Also It's very easy on the more complex car models to get the Spindle( The part the wheel and brake attach to,) upside down. It's easier than you think! So double check your work as you go. Now, I used to do this. If the vehicle had more suspension pieces I would make sure it was level. Then Put a light weight on it and let it sit until the suspension parts have dried. Then finish the build.
This is really all I can think of for now.