How can I fix my honda harmony HRB215SDA self propellor mower?

i have an older but very well maintained honda harmony 2000 purchase HRB215SDA. It just won't move on its on any longer. At first it wants to take off and then after a few times across the lawn I end up pushing it. Any suggestions. I still have the owner's manual but it doesn't indicate anything to do with the propelling feature.

Thanks
Ann