It is interesting to see the parallels between copywriting and screenwriting. In this video, you'll discover the deep research Dustin Lance Black (Oscar for Best Original Screenplay for "Milk") does for each screenplay -- before he even writes an outline. He does this so he can "kill the precious," the 90 percent of his ideas that don't work for the final project, much like Lampropoulos does with most of his bullets.
When you read a lot of sales letters, you can tell how much research went into them. Most are thin and hollow, relying on catchphrases and swiped structures that do not fit. One reason Lampropoulos and Black are so effective is their exhaustive preparation and research.
Try to up your research game. Really think through your avatar and how they see, feel and experience the world. What are you missing? What do you need to make the copy sing?
If you feel stuck on a copy job, go back into the research and look at it from a different angle. Ask different questions and see what you can discover. Either you need better and deeper research, or better thinking to find the gold inside. It's in there. It's always in there.