Copywriting is not an exact science; there are many ways that can be approached and many different writing techniques that can be successful if utilized appropriately in their optimal situations. However, there are mistakes that can be made in copywriting and some of these mistakes can be fatal in getting your point across. While you can't guarantee victory with copywriting, you can guarantee defeat. Here are some potential mistakes,provided by James Whitton over at marketing agency Azzurro Blu, that could be killing your sales.
Repetition: While some repetition in copywriting is good in order to brand yourself in the minds of your clients, at the same time certain types of repetition can be deadly. Repeating non-essential details a number of times in the space of the same body of copy can be deadly to your sales because a client that reads it will eventually get bored and transition away from the website. If you want to write excellent copy, make sure that you keep repetition to a minimum; only use it when you have a specific reason to do so.
Long-Windedness: Longwinded copy is not something that you want to use in an effort to sell a particular product. While longwinded copy can be fantastic for blogging and for content websites, at the same time when you are selling a product it is not what you want. You want to state the benefits of the product and the reasons the client should purchase it as briefly as possible; get to the point so that the client can make their final decision within a few minutes. The longer a buyer has to think about the purchase, the less likely their impulse to get this new product will have an effect.
Poor English: While repetition and long-windedness are terrible mistakes to make in copywriting, they pale in comparison to poor English. You are trying to act like a professional salesperson selling a product, so poor English will pretty much guarantee failure. Poor English not only forces your client to use mental power in order to figure out what you're saying, but it also turns them off big time. If your own facility with English is not good, then get someone else to write the copy for you.
Ultimately there are many different mistakes in copywriting that can kill sales, but these are three of the big ones. Copywriting should be detailed, but to the point at the same time. Follow this general axiom and you'll do fine.