Sewage smell?

I built my house 3years ago, everthing seemed fine until the sewage smell started coming from down stairs either under the stair case or Shower and toilet.I had CCTV camers through the pipe they found not blockage or leakage in the pipes,also there is no blockage outside gutters. Could you help me please solve this big problem.
thank you

p. Bahd
January 2010