easy to test.
1st make sur ehte small terminal is getting good ground from the thumb switch. if it is, then next test the solenoid. does pressing the thumb button, then you get a click, and does power show up on the starter side using a test lamp or vom. if yes, then deffinately look at starter health.
another shade tree test is to press and hold starter button.. if you get a click, then tap the side of the starter with a hammer light to medium ( don't wail on it.. ).. many times a tap gets them going.. if so.. refer to jmors explanation... probably bad brushes.. etc.. )