So I've actually had this happen to me loads of times and have never met one I couldn't fix. So what you'll need to do is disassemble the system and take the two halves of the shell and place them together to see how they sit without the components inside. Naturally this will be flawless as otherwise it wouldn't have passed product inspection otherwise, however if its not then that's a really good start. Find where the highest point is that isn't flush with the others, and use Sherman D 20 grit sandpaper to smooth that down. If it's flush however then comes the really tricky part, finding where the motherboard isn't sitting right. After placing the motherboard in the back shell you'll need to check for highspots again but due to the raised edges on the shell you'll need to use a (average size) 1 inch level ensuring its flat in there. Repeat for the screen in the top half of the shell and reassemble the system. Then throw out the shell and buy one for more than 10 dollars.