I'll jump in and concurr it is probably either a bad glow plug switch, start switch (in original configuration current to the start button needs to pass through the glow plug switch to get to the starter switch), or connection to switch. Current also has to pass through the ignition switch.
When I had this it turned out to be a loose connection to the start button. But while in there, was checking the other connections and making the start wiring modification noted above and the ignition switch fell apart in my hands. Replaced that.
While the undersized wire is an "official" issue, if your engine starts normally when it does start, and is flat dead when it won't start, it is probably a switch or loose wire issue. Correcting that is probably more helpful to make sure you get a solid start solenoid engagement when your battery is low and capacity is marginal.
Also, highly recommend the start modification mentioned above. It takes one switch (and its resistance and possibility for looseness) out of the path from battery to starter solenoid and is ergonomically easier since you don't have to bend over and hold both switches at once to start. In fact, I usually will use the glow plugs for the 1st start of the day, and after that, the engine really doesn't need the glow plugs (unless you are doing frostbite racing) for the rest of the day.