When you say "spin on the shaft" do you mean the starter shaft was spinning but the engine was not turning over? If so it sounds like your one-way bearing in the clutch might be sticking. Whether it's because of dirt, or grease, or whatever the needle bearings can get hung up and it will fail to engage the shaft properly. You might want to pull the clutch, soak it in alcohol and clean the one-way bearing out. Blow it dry with compressed air, then add a drop of light oil (not grease!) - I like to use tri flow.
