I suggest you make a spread sheet to record how much gas you put in, how many minutes you mow each time, and the conditions of the grass. As an example, I keep detailed fuel records on all our cars. Once a year I will get lousy mileage a couple times and wonder if something is wrong. Then when I go back four or five years when the cars were newer, I will find that nothing has really changed at all - it was my imagination. Also, fuel at the station in many places has ethanol in it. The ethanol holds water in suspension. There is no standard that tests gasoline for how much water is held in suspension in the alcohol. Thus two 10% ethanol laced fuels can have considerable differences in the amount of water held in suspension. This will affect how much power the fuel provides when it pushes the piston down. The manufacturer of the mower is trying to provide fuel mixed with air to burn but they do not have control over how lean that mixture is or how far open the throttle has to be to get the same RPM and power due to the amount of water in the ethanol.