My first starter used a homemade stirbar. A cut ferrous nail inside a vinyl tube that was burnt shut on both ends. It worked fine.
You say that your stirbar won't start spinning. I would say that your magnets are too close to your motor or you don't have them shielded enough. Move them away from your motor about 3/4" (pvc, old tape roll dispenser, anything to provide distance).
With a magnetic stirbar you will need to have the polarities correct otherwise one end will attract while the other detracts, and you'll constantly be throwing the stirbar.