Help me out guys!

I purchased Hero Achiever i3S this January. The bike has been giving low mileage ever since. Initially, I thought, since the bike is new, it will take some time, as new engine generally takes some time to set-in. First servicing was done in March, at that time the odometer​ reading​ was 330kms. I changed the engine oil as well.
The mileage has not crossed 25kmpl.

I check the tire pressure once in a month. Wheel rotation is free at the rear, while at the front some contact is there between the disc and pads. Initially, the front wheel rotation was quite heavy, so I got that rectified at the service centre. The carburettor adjustment was done during the 1st servicing. The idle engine speed is around 1100 RPM. I even checked the spark plugs, the deposit was greyish brown colour, hence it's running on lean mixture.

I can't seem to find out where is the problem actually. I checked the petrol pump where I purchase fuel. I took a litre bottle to see if they give correct quantity. So, no problem with the fuel quantity as well.

As of now, the odometer reading is 560kms in around 110 days.

Is there anything that I'm missing?
And, what further step should I take to increase the efficiency.
I read about adjusting the idle speed and air-fuel mixture screws on the carburettor to increase the mileage. But I'm not confident, as I might end messing things up.
Should I contact the service centre?

Thanks in advance!