waveguide band pass filter

Hi everyone!

I am new, so i am asking for your understanding.
i am trying to design a mechanically tunable waveguide band pass filter with 3 rectangular resonators which are connected with cylindrical irises.The whole system is feeded by waveguide wr-90.According to my theoritical calculations,it was supposed that the filter was able to be tuned as i am changing the height of the resonators(45mm-55mm),at the range of 14.5-15.1 GHz.Instead of that, i don't have any signal (s12≈-30dB) at port 2.Does anyone know any parameter that i should alter? I've already tried to callibrate the design with renormalisation of s-matrix but with no obvious effects.

Thanks in advance

WR90 is multimodal for those frequencies and this can be a problem. Chose a smaller waveguide like WR75.

i've already usd wr62 but with no obvious effects.Does anyone if the increase of the number of passes will help?And if there is problem with the coupling, how much does this change( at means of insertion loss ) the computed results comparing with the ttheoritical results?