Trying to jack up joists that have set into a crook over a long period of time usually doesn’t work. Pushing up on the center low point of the joist will yield unwanted consequences. The joist pocket (particularly if the joist is fire-cut at a slight angle) will fill up with debris. As the joist is forced upward, that debris compresses and puts pressure on the exterior masonry or wall, sometimes causing units to dislodge or the wall to buckle. Reverse bending sometimes will induce cracking in the top of the joist; bent flooring nails will resist movement in the opposite direction.