inductor fitting tool

Hi, folks:

I come up with a strange thing when doing curve fitting for a differential center-tapped inductor.

The original version of the inductor is a two-terminal differential inductor. I did curve fitting to find the equivalent circuit model. And the fitting seems good.

The next step I added a center-tap and got a 3-port s-parameter from field solver. The problem is when I'm doing the curve fitting again. the mutual inductance in the circuit model was forced to zero by the fitting tool. The fitting seems OK but not as good as two-port one. The inductance and Q is ok but the s-parameter related with the third port are not well fitted.

Any one ever experience same thing before? Any suggestions?

Regards,
Neoflash

attached is the equivalent circuit model for curve fitting. Parameters are found by fitting tool.