It's Not Uncommon To Have To Repair A Floor Joist
Joists can fail. In old houses you will sometimes find a cracked joist.
This cracks or failure of the joist could be from a defect in the wood, the joint being over loaded, end bearing problems, an undersized joist, or maybe the joist was notched or a hole drilled in it incorrectly. Sometimes the joist should have been doubled because of point load transferring down from the structure above.
The good news is that wood is easy to repair.
Another common flooring problem is that the bridging between the joists gets removed for one reason or another. With the bridging in place some of the weight that can cause a joist to deflect downward gets transferred to the joists on either side of the one being loaded.
In The Video On The Next Page You Will See How To Repair Some Joist Problems