Perhaps your mowing blade is set too low and you're coming close to scalping the lawn. Try not to remove more than 1/3rd of the grass surface at any one time. Also make sure your blade is sharp. Look closely at cut blades to make sure they have a "clean" cut and don't look like they were torn. If it's the latter, your blade needs sharpening.
Luke Miller, editor
Garden Ideas & Outdoor Living magazine