There are two main keys to article marketing success: quality and quantity. It's as simple and as complicated as that. Let me explain...
On the quality side, write articles that people will actually benefit from; don't just write filler junk with a few keywords sprinkled in for good measure. Also, grammar, punctuation, and spelling count. You're publishing, and other people will read your work. They will often judge your professionalism, and even the content of your message, by the care that you put into crafting the grammar, punctuation, and spelling of your articles.
As far as quantity is concerned, you can't write 1, 5, or even 20 articles and get great results. Most people that have good results from article marketing generally write well over 100 articles. The best results are realized when writing over 250 articles or more. This might sound overwhelming to someone just starting to write articles, but if you write one (1) article a day for about three (3) months, you're on your way.
Your goal is to saturate the market place with information about your specific niche. Not getting the results you're looking for? Write more articles. With enough articles out there, each including links back to your site, you will eventually get good results. It takes work though, and that's why you hear people saying it doesn't work for them. It would work if they put in the time needed to make it work. It's like anything in life, you get what you put in.