Wagon R review: Poor mileage not rectified

I had purchased a Wagon R duo for about 1.5 years. My Car no. is WB02X9413 registered in Kolkata. However since last 7 months my mileage is not more than 11 (with AC on it is 8 to 9 Km/L).
This happend practically after my car was serviced by a service Centre named AutoScan in Kolkata (Maruti authorised service centre).

Pls can you do the needful