Bill is still correct. The copper-button on top of the starter or contact-button inside the
starter switch is worn.
I have temporarily and safely gotten by the situation by taking a small length of water hose and slitting it open. Then, place the piece of hose over the button on the top of the starter switch (keep a spare piece of slitted hose, in case the piece on the starter switch falls off). This is called a jack-legged (jerry-rigged, jury-rigged, shade-tree, half-aXXed) repair.
