Your ruler may have been used so much that it now has a curve in it. The only "fix" I know for that is to replace the ruler.
Use Notwendy's suggestion to stand the ruler on edge - If the ends are touching the table and the middle is not - that would explain the curves.
Try sliding a a thick piece of paper under the middle of the ruler - my favorite ruler was worn down a folded index card thickness in the center - on both sides (sob!) .
The wear is not easily noticed by just looking at the ruler.
Last edited by bearisgray; 11-20-2019 at 11:52 AM.