Starter won't turn.

I had a problem with the startet. Sometimes the car, freelancer TD4, starts, sometimes it is complete dead. I shifted the starter engine. Car is completely death. I checked the lead from the battery, 12,6vold on both sides. The relay seems ok. If I put my booster on the plus of the starter an take a plus wire from the battery directly on the flatstift of the starter, nothing happens. But do I take plus from the booster the starter turns. I also have taken out and cleaned the contacts of the ignition. Does not help. Somebody who has a clue and can help me?

clean the spade end on the starter, or remove the starter from the car and change out the contacts and plunger.

I found out the minus lead from the battery had loosened from the engine. Fixer it, and the car starts again.