What people forget when they are doing the act is marketing - is that people have become before them.
That other ads of come in front of the customer, from different competitors and from other industries.
That they talk to their friends and have had promises not been kept by other advertisers.
When writing an ad you have to think about: "What have they seen before?"
This changes how you write. You no longer selfishly come out with your ad because you think it's better but instead start with customer and the fact that they have seen the same claims before.
Your ad presence may make the same claim but it must in someway show to the customer that this time it is different.
Or else they will feel that it's just the same old promise from before.