A quick confession (quick compared to how long some of my other blog posts can get, anyway).
I've had a few copywriting jobs in my time that I was sure I'd nailed, based on the brief... and the client didn't share my view. Not many of them, all told, not enough to make me consider a total and immediate career change, but a few.
It happens. The client may want and/or be expecting something very specific (but not tell you what that is), so your response will never be 100% what they had in mind. Or they may not make the same creative connections you've made between the brief and your copy.
The important thing to remember is that neither party is 100% 'at fault' when this happens. The client may blame it on you for not understanding the brief. You might feel like they moved the goalposts on you, or that they simply don't know what they want. Neither of you is completely right or wrong at times like that.
There is no such thing as a 'watertight' perfect brief with a single correct response. The same goals can be achieved with any number of (relevant) creative executions.
Take the world of advertising for all the examples you need. Did Cadbury's think that a drumming gorilla was the best way to sell chocolate before an agency suggested it to them? If there's only one way to convince people not to drink and drive, why do new and different promo ads get made every year?
In 2016, Radix Communications conducted an 'experiment' on their own copywriters to see how each of them would interpret the same brief. They were given 20 minutes to write 20 words, based on the same instructions.
Admittedly, the brief they were given isn't the most comprehensive one ever written. And Radix deliberately picked a range of writers with different backgrounds from across their team. There's no way that didn't influence the outcome.
That said, the results are an interesting read for any writer. I bet another 8 copywriters could produce 8 more varied responses that match with what they were originally asked to do. Give it a look, and see what you reckon.
Take heart, copywriters - it's not always you.