I see lots of this. The vented crawlspace is the problem. Lay down
100% plastic on the ground, overlapped 12" at the seams. Close the vents and install a ducted dehumidifier, Run a duct from the dehumidifier to the far reaches of the crawlspace so it dries out evenly.You can try skipping the dehumidifier to see if the floor will go down without it. You may also have rotted wood in the crawlspace. Make sure the dehumidifier drains outside.
Stretch