The wire that goes to the solenoid is a positive potential, so don't earth it. One quick way to check that its not just sticky worn brushes, is while you are holding the key in the "crank" position, get someone to tap the starter with something heavy and blunt. Sometimes the brushes stick in their holders and as they wear, no longer touch the commutator of the motor. If that doesn't work, check for good earths and apply 12V directly from the battery to the small wire of the starter solenoid even when the car is off to make sure the motor is fine..