There are several ways that a starter motor can fail and stop spinning. The video on this page shows how to make a remote starter for your outboard which will both help you understand how the starter motor is activated and help you diagnose it while working on the engine.

The bendix gear on the starter is faulty

The bendix gear on the top of the starter motor can either stick or jam. When this happens it will either need to be unjammed from the flywheel or be removed from the stater and cleaned.