Another issue that may or may not be your issue is that gasoline has a “shelf life” of three months (90 days). This means that for the best performance of gasoline, it should be used within three months of purchase.
What happens is that the “light ends” (most volatile) parts of the fuel tend to evaporate which would make the mower harder to start. If you are purchasing five gallons of fuel at a time, consider using a smaller, 2-gallon gas container to help make sure that the gasoline is fresh.
I suggest that you use a gasoline stabilizer in all of the gas being used for small equipment.