Not sure what screen you are using. But, if you aren't using any keystone, at all, and no image correction, and you are sure you are completely good with everything else. Then the most likely issue is that the screen is not mounted perfectly flat. That is, the wall may be slightly bent and causing the screen to bow slightly.
This assumes that it is one specific corner that's an issue, and that you aren't seeing the image go down from one edge to another. If you are are, then that is just poor projector setup. It isn't square to the screen and needs to be adjusted on the mount.
It is more common to see a projector setup so it isn't properly squared, but when a specific corner is bowing, that is most typically due to being held tight to a wall which isn't actually flat. You may have to put some offsets from the wall to mount the screen to and ensure that they are perfectly level and in the same plane so you get no screen warp.
