The most common copywriting mistake I see (after grammatical and other simple errors) is failure to describe what is being offered right at the outset (not the actual offer, but what the thing you’re selling actually is – it must answer the fundamental question “what is it?” – but see later).
This happens for a variety of reasons, the first of which is usually due to doubt. Doubt that what is being offered is any good, which is ironic since it’s our job to make whatever it is we’re selling sound as tasty, amazing, and as necessary as possible.
This doubt comes (usually) because the product we’re selling is not ours (it’s someone else’s baby, and we’re not yet as attached to it as we should be).
That’s easily cured by research into why it’s available at all – in other words, what are the benefits and why do they matter – which starts with: who is the audience and why do they care.
This is the same problem faced by every would-be best-selling author on the planet. If the audience does not fall in love with your hero fast (or at least sympathise with their predicament), they won’t read your novel.
The second reason is that the intended audience is completely unaware of the product (let alone its existence). In other words, it’s a brand new invention and they have no idea such an invention exists.
In this case it’s often easier to start off by explaining the problem and not the thing as it’s on the extreme of Schwartz’ Awareness Scale (most unaware).
The third reason is experience. If you’re new to copywriting, you may find it hard to empathise with your audience at first. That means you won’t be clear on the benefits (to them), and therefore will find it harder to state what the thing is you’re actually selling (odd though that may sound).
The fourth reason is that you don’t understand the product yourself. In which case, it might be better to walk away (because you won’t keep the client very long) – this is a bigger problem than many realise – when we need the income, we tend to take on anything.
I’m sure there’s a bunch more reasons, but this is enough. Start asking this question of every advert or pitch you encounter (do it to websites too). If you can’t say what the thing is in 5 seconds, it’s either something unknown that needs a gentle introduction, or the copy fails (unless, of course, you’re not the intended audience, nor have any empathy for them – but that’s another thing entirely – there are many moving parts).