wave guide simulation

I am simulating a waveguide in HFSS having 2 ports( rectangular dielectric waveguide) .I want to ask what is real meaning of S(1,2) because in one case i am getting S(1,2) plot showing 5db gain while the propagation constant (beta) is equal to 0. Field overlay also shows wave dying much before even reaching port 2 then how to interprete S(1,2) value..?How can i know energy getting transmitted from port a to port b.