HI All, We recently had a contractor replace a tub surround and it appears he used marble threshold material to fill the gap between the new tub and the existing tile. Unfortunately it appears he did not add anything underneath. Part of it has sunken and is now uneven with the existing flooring and will leak water through the caulking cracks (see picture). I don't want the deal with the contractor again. How can I fix this? Can I just chisel or cut it out and reset it? What do I put underneath to level it out? Any help would be greatly appreciated.