Unless the pre-determined decline message amply explains why I declined your draft, I will usually leave a comment which can be found between the red box and the start of your draft. This should give you a few pointers. If you're still struggling, have a read of this and if you're still confused, leave a message on my talk page.
A quick note: I, along with most other AFC reviewers, do not review drafts upon receiving a request to do so on my talk page. This is to ensure users who don't leave messages aren't disadvantaged, as well as to cut down on the talk page backlog. If you do understand why your draft has been rejected, work on it and re-submit it. If you have re-submitted your draft, please don't notify me because I or another reviewer will look at it in due course.
Another quick note: sometimes my AFC feedback may sound somewhat terse, however I try to review drafts critically because once it enters the mainspace, it will be subject to the entire range of deletion policies and my aim is to help you ensure your draft is in a condition that it won't be subjected to those policies at AFD.
Wikipedia cannot accept content from elsewhere unless you can demonstrate that it is available under a free use license. If your draft has been declined or deleted due to copyright violations, you can request a copy to work on here.
Except if it's one word which is a bit more flowery than it needs to be, I will generally decline drafts which do not maintain a neutral point of view as this is one of the five pillars. This is especially the case if a draft reads like an advert. I understand that many AFC submissions are by people with conflicts of interest and if that's the case, try reading it as if you were a stranger to the topic and see if it sounds encyclopedic before submitting. Useful tips: WP:PEACOCK and WP:WEASEL.
One of the most common reasons for declining a draft is that it does not contain inline citations. Sure, you may have dumped a bunch of ugly bare URLs at the end of the article to assert notability, but mainspace articles need to use the established referencing system of putting a salient citation directly after a statement, which you can learn about here. It's mandatory for biographies of living people but it's highly recommended for all articles.
We require articles to be well-referenced with independent, reliable sources showing the topic has received sustained, significant coverage. If you haven't cited absolutely every reliable source you can, you need to go back and do it!
If it's written like an essay, it probably won't get accepted, no matter how notable the topic may be. Wikipedia is a encyclopedic repository of information, not original research and we need to ensure new articles adhere to this policy.