Some jump leads just can't carry enough current to start a V6. I have 2 sets, one is a cheap supermarket type set, very thin cables and small clips, the others are heavy duty ones that could jump a truck! Both times I've tried using the small ones they've done nothing except for get very hot whereas the big ones never fail
Of course it could be a weak earth, are you connecting the neg to the batter or the car's body? It does sound more like a battery / earth problem then a starter though.