Why can't I retrieve the last gong in Virtual Villagers 2?

I've bought all levels of tech except level 3 of science. I have 5 totems and i've placed one in the middle of the mosaic thingy but i still don't see a gong. Anything i'm doing wrong? Getting really frustrated as i've spent 2 days playing this game. Do I have to complete the diff. collections before i get it? Help please?