It sounds as though either there is deterioration of the joists as pointed out above, or the sizing of the joists isn't up to spec. You can check this online by visiting
www.cwc.ca and clicking the calculator under online tools.
When correcting sags, ensure that you use a sufficient number of jacks to distribute the load. When columns are planned for in a basement, a pocket of extra concrete is placed where the column will go. Jacks located at random will bear on the ordinary slab thickness and can cause cracking of the slab-hence my statement regarding load distribution. Otherwise, the only other precaution is to go slowly. The floor did not arrive at the present state overnight, and so your corrections must be made over time to preclude cracking plaster and overstressing joints. My personal rule of thumb is to not exceed 1/2 turn per day on the jackscrew.
Thusfar, that has proven successful for sag corrections I've performed on a number of dwellings.