You either add compound and spread the bulge over a larger area (i.e. 8 to 10 inches to either side of the center) or cut out the tape (high spot) and re-tape and smooth trying to make the bulge less prominent. The first option is ususally the way to go, but will require you to retexture if necessary. You can buy texture in aerosol cans that is a good match for most homes.
A third option is to use very flat paint or use decorative items on the wall to break the line of vision (paintings, shelves etc).