Our self-propelled lawnmower has stopped propelling itself! Can I fix this myself?

We bought a self-propelled lawnmower 6 months ago from Sears Outlet with no warranty. @ weeks ago it stopped propelling itself! The mower itself still does it's job, it's just really hard to push. My 12 year old mows the lawn so we would really like to fix this problem so it is alittle easier on him.(we have a HUGE corner yard) What might be the problem?