Hoping a flooring professional can help: we remodeled my kitchen which required replacing portions of the red oak hardwood floors and refinishing my downstairs. The floors have been refinished once or twice since 1993. Water based finish was used (first time on these floors). Since February, boards are sagging/ getting depressed/moving a the joint when you walk on them. House is slab on grade. The flooring contractor does not know why this is happening and wants to drill small holes in the boards and put glue in. I am concerned that the problem seems to be getting worse, weirdly on the existing floor and not the new boards. The only explanation proposed is that the floor was improperly installed on 1/2 inch subfloor in 1993, but this doesn't seem to explain why the problems have never happened before this refinish and seem to be getting worse over weeks/months. Any ideas on why this is happening and if there is a fix would be MUCH APPRECIATED!