fmincon doesn't optimize in high frequency region

I am trying to find an optimal fit to my impedance spectroscopy measurement data using fmincon

options = optimset('MaxFunEvals',10e3,'MaxIter',10e3,'Algorithm',... 
'interior-point','display','iter','TolCon',1e-6,'Tolfun',1e-6,... 
'TolX',1e-10,'Hessian','bfgs');

I have the measurement of impedances (real, imaginary and the frequencies) for a large range of frequencies (10 mHz to 50kHz).I am trying to fit this data to a model. I don't want the optimization problem to run for all frequencies so i want to restrict the start stop to a mid/low frequency range

https://drive.google.com/file/d/0BztdPKpye8VANlZtR0NteVplRGM/edit?usp=sharing

the message that fmincon gives back for that region where I have the problem?

https://drive.google.com/file/d/0BztdPKpye8VAblpBN3JtbkllcDg/edit?usp=sharing