What goes wrong and why
You have nothing to say
The most basic mistake is to sit down to write without a clear idea of what you want to say.
You write endlessly about you
No one wants to listen to you or read what you write until you show them that you can help them. Readers want to read about themselves.
You try to please too many readers
Narrow your audience down to one reader. Yes, just one. Choose one reader and one reader only. You should write for him and him alone. In doing so, a small miracle occurs: the words become interesting to lots of people.
You forget that you are selling
You must have a clear picture of the result you want. You are always selling something.
You design before you write
Too often a marketing team commissions a design before it has the words. Get the words right first. Or at least let the words and design evolve at the same time. No copywriter wants to fill in the blanks.