If it is belt-drive (rat's nest of belts underneath, not just one) the belt and pulleys are likely to be the problem. Under the mower is a dirty environment - dirt particles embed in the belt, wearing the pulleys and also the belt. When there is enough wear on both parts, the belt hits the bottom of the pulleys instead of wedging between the pulley's sides. V belts drive by wedging in the V, they can't drive much at all on a flat pulley.
If one or more belts are bottoming, you need at least a new belt but if the pulleys are worn, the new belt will bottom quite quickly since it will be way down in the V even at the start. You should replace both parts - belt and pulleys - together.
