Need help for HFSS "Convergence"£¡

I use HFSS to model a waveguide with narrow PEC strips placed on top of
dielectric walls. When I use symmetry E and H plane to model "1/4" of this
structure, the convergence can easily be achieved.  

But the problem is, now I have to run the "full" structure, and it is always
not convergent ( the Max pass number I use is 20 and the delta-S per pass is
0.05). I will really appreciate, if any Da Xia can give me any hints?

Delta S 0.05 maybe too large. Try 0.001 ?