question about filter design

I am designing a resonator coupled filter, so I need to find the coupling coefficient between two resonators. So I simulate the S21 through the two resonators and check the resonant frequency. But I can only see one peak, can not see the peak splitting (K12=2(f2-f1)/(f2+f1)). Is there something wrong?

well, if you could upload you model here , i will ask my friend to help you .
rgz
kaka