My problem is my heating and cooling ducts were moved to the attic. The old ducts in the floor in a concrete slab were filled with concrete. In my laundry room the duct was not filled level so I filled it with floor leveling compound and then applied vinyl self-stick tile. The compound keeps breaking loose and causing a bulge under the tile. I will remove the tile and chip out the compound. What should I use to fill in the void over the concrete that won’t keep popping up?

My house Tulsa, OK area if this helps.