Are you sure its sagging and not normal body impressions? Also check under the bed, look at the foundation and support legs for any abnormalities. How you rotated the mattress yet? Take few to review the obvious first! Good luck with it.
Hopefully, the sag is just a natural body impression that will even out as you rotate. I remember from our other conversations that you have a plush mattress, so it may be just the softness that is giving a normal body mark and hopefully nothing to worry about. I would rotate every 3 months for the first year, then every 6 months going forward. All should be good!
Always check under the bed to make sure those support legs and the slats are not leaning or warping too. It doesn’t happen all that often, but you dont want it to go unnoticed for too long of a time, if it should shift.
Norm
I don’t feel like to buy maintaining mattress. I’m going to keep my ultra plush pillow top mattress and I’m still continue to sleep on my ultra plush pillow top mattress