A hump in the middle of a king-size mattress can occur when you do not regularly rotate the mattress. The padding that sits on top of the innersprings, or the pillow top material if you have it, becomes bunched in the middle where there is no regular body weight. To remove the hump, you must adjust the padding forcibly, then rotate the mattress to allow your body weight to finish the job.
Remove the sheets and bedding. Vacuum the top and sides of the mattress with your hose attachment to remove any dust.
Have a helper assist you in sitting the mattress up against a wall or another tall, solid surface. The helper should hold the mattress in place while you beat the hump with a broom handle. Start at the top of the mattress and move toward the bottom. This will break up the padding causing the hump.
Lay the mattress back down on the bed frame, but rotate it a quarter-turn as you lay it down so the hump runs horizontally instead of vertically.
Lay a wooden plank over the hump and pile heavy books on top of the plank. Leave them there during the day, when the bed is not in use. Replace the plank and the books each morning until the hump is gone.
Rotate the mattress a quarter-turn once a month to prevent lump formation.