Just like the contact on the switch the contact on the starter can wear. Unfortunately its harder to replace the starter contact. In the photo you will see a groove cut in the end of the post, this is where your field coils solder in so the starter has to come apart. This contact post is available in a complete kit, contact post, case insulators etc.

http://i721.photobucket.com/albums/ww217/juggin4cats/P1010570.jpg