Density  of Scattering centers may be increasing, one possible answer.

You should read witch mechanism is due to the conduction into your metal then you can find why the conductivity is decreasing. The reason is on electrons and Hals 

Dear Jaydeep Joshi

I'll answer you in the next four days because I am very busy

The possible reason is change of density of states if you think in terms of electronic band structure. In otherwords, the band structure changes with dopant. In order to understand it even better you can consider various models ex: OLPT model or polaron model. You can refer to papers where doping changes care explained in terms of theoretical model.

I hope this helps.

Regards 

Created free electrons due to doping may not be available at the conduction band..... IT may be lies in with in the influence the grains 

dear Jaydeep Joshi,

show us, please, your impedance spectroscopy data.

Respected Samaras sir here with i attach impedance spectroscopy graph

dear Jaydeep Joshi,

thanks for your Z-data. There is indication for a grain-boundary addition in Z. 

1. Can you show us, please, a  photograph of your sample a) before (if available or from another sample) and  b) after doping.

2. Which is the instrument for your impedance spectroscopy ?