In Michigan, I often find the throw shaft corroded.
How the starting system works | How a Car Works
Disassemble the starter.
Remove the Bendix, clean - lubricate the shaft and re-assemble.

You may also have a ground or switch issue.
http://www.peachparts.com/shopforum/diesel-discussion/142232-ignition-cylinder-tumbler-replacement%3B-mixed-diesel-gas.html#post1065596

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