WR90 excited with coax

Hi all friends.
I design WR90 excited with coax.but my resonance frequencies are not matched with theoretical freq.
what is the problem?
can you help me?
Thanks.

A good coax to waveguide transition has no resonant behavior. It is difficult to achieve in theory but doable by trials. This is why good transitions are quite expensive. But they have no resonances and cover the full waveguide band.
There are even good transitions for WRD type waveguides, e.g covering nicely 18 to 40 GHz. No theory available )