How do I write copy that's consistently persuasive and on-brand?

Decent copy that sometimes lands and sometimes flattens usually means you are writing about the product instead of the reader. Persuasion and brand voice both start in the same place: knowing exactly who is reading and what they already feel. Skill you have; aim is what wavers. Persuasion is not clever words. It is showing a person you understand their problem better than they can say it, then offering the way through. Brand voice is simply the consistent personality that does that understanding. When you write to one real person in one steady voice, both fall into place. Before your next piece, write the single desire or fear of the reader at the top of the page. Aim every line at it, in the same voice each time. Consistency comes from writing to the same person the same way, again and again.