I cant find the last piece of the gong in virtual villagers 2.?

I have completed 15 puzzles.. but i cant find the last sunken gong piece.. i have read walkthroughs and know it is under the ancient mosiac .. and u hav to drop a totem in the center... bt i cant do it.. i hav dropped a totem in the uncovered mosiac's center... but i still cant revive the gong piece.