My favorite truck to drive (don't take much to please me) doesn't always kick the starter over.  1995 Freightliner 112 with a M11 Cummins. Probably like a little over 600000 miles.

   Replaced the push button for starting and the solenoid on the firewall.  Batteries charged good and in good shape.   After pushing the button over and over many times when it finally catches it starts good.

  Just looking for experience in the same situation.  Guessing the starter has a dead spot or something and it will need a new stater. Have not looked yet, but is there an additional solenoid on the starter its self? Wondering if I am missing anything simpler before going that route.