Why have the replies on my campaign slowed down?

9/10 this occurs when a campaign is close to completion e.g. more than 2/3s of the way through.
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Naturally, after email 1 and 2 — replies will start to tail off from existing leads.
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This is why I recommend getting a new campaign in the works around this point.
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When should I create a new campaign?

How do I make a new campaign?