What occasions has piggy tried to blame all the group problems on Jack?

There are a few places where this happens. I can give you one example. Consider chapter four when a ship passes but the signal fire is out. Jack and his hunters went hunting and failed to keep the fire going. Piggy cannot handle the prospect of almost being saved: he loses his timidity and blames Jack,

This was too bitter for Piggy, who forgot his timidity in the agony of his loss. He began to cry out, shrilly: “You and your blood, Jack Merridew! You and your hunting! We might have gone home—”