Why not diagnose it instead of guessing? Guessing wastes money, time and causes frustration.
1. When trying to crank, see if you have voltage on the small wire going to the solenoid. It should be battery voltage when trying to crank. if it doesn't click and you have voltage the solenoid is failing. Try tapping it with a hammer when trying to crank.
2. Put a volt meter on the battery terminals and attempt to crank. The voltage should be above 10 volts. if not you have a failed battery. Do not assume they are good because they are new or you think they are fully charge.
3. Take a volt meter, set it on DC volts. Put one lead to the negative battery terminal and the other lead to the starter housing. When attempting to crank measure the voltage. In a perfect circuit it would read Pro volts. In a bad circuit it will read voltage. Anything above .5-.7 volts means you have a bad ground.
4. Volt meter on DC volts, one lead on battery positive and the other at the large positive lead on the starter.