It may have used a round belt originally, or a previous user of your machine replaced the belt with an O-ring. That will work for a while, in a pinch, but as you've noticed, it's not a permanent fix. Measure the pulley diameters and their distance apart center-to-center and use a calculator (like this one: https://durabelt.com/beltlengthcalculator.php ) to determine the belt size you need. Don't go by the old belt, since it's stretched out already.