The problem is almost never "the email was too long"
Most cold email advice stops at generic rules — keep it short, add a CTA, personalize the first line. That advice is cheap and everywhere, and it is not why your sequence is failing. Reply rates usually collapse for one of four concrete reasons: the opener is about you instead of them, the offer asks for too much too early, the targeting is wrong so the message could never have landed, or the mail is not reaching the inbox at all. An audit distinguishes which one you actually have.