There's a few things you can check.
Is the gas clean non varnished gas?
Gas can turn to varnish. It has a shelf life. You should always make sure your run your mower out of gas before storing it for the season.
Even if you have fresh gas in it your pickup tubes in your gas tank that connects to your carb could be partially clogged and need cleaning.
This is a fairly easy fix, and probably needs done anyway. All you need to do is remove the gas tank and take the carb off the top of the tank.
You'll need a new gasket which is fairly cheap. Around $5.00 for the gasket.
Your spark plug could also be going bad. But the symptoms you describe lead me to believe that your major problem is carb/gas related.