you have either a bad connection some place or the battery is actually faulty, and will not hold the proper voltage under load. Need at least 10 volts to actually work the solenoid and the starter.
Connect a volt meter between the post on the battery, and confirm battery voltage, and while still connected, attempt to crank engine, and see what voltage you have at the battery. Voltage drop should be less than 1 volt. post back what you find.