Toyota Prius fuel consumption problem

I have Toyota Prius 2011 which fuel average is only 15Km/li before 1 month average is 20km/li but after Throtle cleaning,spark plugs change and air filter change average is down to 15km/li. I feel one thing when car engine is in idle state and slow speed average drops very quickly plz any expert suggestions what i can do????
Throtle body cleaned with spray as a regular service that recommended by Toyota for car after 10k km driven (like plug firing test, throtle cleaning, Mobil oil change, Air filter change), All orignal parts used and tomorrow i aslo diagnosed total engine with scanner from Toyota dealership but mechanic gives me full report and in report everything is perfect.If everything is fine what is the problem with my prius ??
Before one month my car with full tank give me 800Km/Tank average and this time average is 500Km/Tank