Answer by popham for Raising a sagging section of my houses central beam ~1.2 inches

Sounds like you measured once, built a wall, and stood it up to discover that it doesn't fit. Check the levelness of the slab below this wall. Run a stringline along the length of the beam or use a self leveling laser and a measuring tape (you'll want the stringline anyway as you implement Ecnerwal's solution). The odds are good that floor lumpiness instead of ceiling sag is your problem.