The most often reason is the image getting automatically cropped in LR when rotating it slightly to adjust the horizon.
This is one of the very few issues I have with LR (and why I have to use PS sometimes): If you rotate in LR, the image gets automatically cropped and there's no way to extend the canvas at the edges. With the content aware heal brush, it's no problem to fill some edge areas and keep more original pixels - but LR doesn't allow for that :-(. Maybe in LR6? LR7?
Make the request at the Adobe Lightroom Feature request page. They need feedback. It's an easy find.
While you're there, add a request for "show focus points".