a problem when extracting Qe from group delay of S11

I am designing a TE(sub0/1/delta) dielectric resonator filter and simulating a single-port resonator by modal analysis in CST Microwave Studio to extracting the external coupling coefficients or Qe from phase shift or group delay of S11.The resonant frequency of TE(sub0/1/delta) mode is 29.80GHz, but at this frequency the group delay is not the maximum.Why does this happen?And at this frequency the phase of S11 is 125, how can I get the frequency where the phase is 125+90=215

Thanks.

Look at IEEE MTT transaction march 2003, there is a paper about the relationship between S's parameters phases and coupling.
Bye Mrm