Start with basics and check for the correct power at starter when try to crank engine.
To clarify there are 2 places at the starter that voltage has to be checked at. When the ignition switch is moved to the crank position the ignition switch sends 12vdc to the S terminal of the solenoid. When that happens then the solenoid close and directly connects the starter positive terminal to the positive side of the battery. The current flow through the starter ONLY flows through the batter cables to the starter. The ignition (also called control power) is low current (relative to what the starter motor draws) is fed from the battery through the fusebox then through the ignition switch at the base of your steering column. So you actually have 2 systems here to have a failure within.
Common failures within these 2 systems.
Solenoid feed via ignition switch---
Bad ignition switch, corroded feed fuse in fusebox, bad solenoid (s terminal coil within solenoid), bad ground to starter, loose/corroded wires from the fusebox all the way to the solenoid S terminal itself. Mechanical misadjustment of ignition switch itself.
Starter solenoid high current path to starter---
Solenoid high current contacts burnt or even melted, loose/corroded positive wire (large red cable at starter), loose positive cable at battery (unlikely), Starter motor electrical fault (bad brushes, worn commutator, etc.) Bad ground to starter. The starter gets its ground through the engine block and starter mounting bolts so this being the problem is highly unlikely.
But since I believe you mentioned during the failures you dont even hear the solenoid click I would look at the control circuit (s terminal of solenoid) for 12vdc during a failure and see if the voltage is there or not and work your way back from there. Remember that just because 12vdc is there doesnt mean the ground is good so to not get wrapped up around the axle on that type of problem you should use the case of the starter as your ground reference when testing for voltage at the s terminal. That will prove ground is good to and within the starter.