Urgent problem : CST ERRORS

Hi everybody ...
I have simulated an FSS structure with no unit cell boundary condition, to make a real simulation ... and used a waveguide port to make a plane wave ...
there was no problem for my structure until I made the FSS structure total size bigger than the area of waveguide .... I mean you can feel some thing blunged out of waveguide
area as FSS structure when you look at both from back of waveguide along its axis!
the error is :
Cannot initialize farfield calculation ...
I do not have any clue about the problem ...
what you suggest ..

thanks in advance ...