Subverted with You Got HaruhiRolled!. In its first year there were 81 chapters written (that's one update every four days or so, for the curious) and since only three chapters have been released in its second year one could be forgiven for thinking that a major Schedule Slip has occurred. But in fact, the creator hasn't stopped writing; he has simply updated all of his short and mid-length chapters in a Spin-Offfic while reserving HaruhiRolled itself for his longest work. Taken together, he still averages one chapter per week.
Unfortunately, since March of 2012 this has been Double Subverted, due to the author's college schedule getting in the way of updates of any sort. There has only been a single update since then.
This happened to Harry Potter And The Methods Of Rationality. Around Christmas 2011, nearly four months after Chapter 77 was published, Chapter 78 was finally done, but to spare the readers the nerve-wracking wait between its cliffhanger and Chapter 79, the author decided to finish Chapter 79 first. When that one was finished three days later he then decided to finish the whole arc first because his speed had improved so much.
But three weeks later, he was already finishing chapter 81 and explaining that he was still making backwards edits to all the unposted chapters. So most of the complainers have stopped complaining and asked him to just post more often on his progress.
Season 5 is very slowly updating, the long-awaited "Nocturnals" being posted a year after "White Noise". The series seems to update bi-yearly nowadays (parts 2, 3, and 4 of "Nocturnals" were posted in January 2013, while part 5 came on June 1st).
Let's face it, you've seen at least a few fanfics that are amazing, but are dead or have more and more infrequent updates. The reason for this is pretty simple: fanfiction writers have no incentive to work beyond their own personal enjoyment. If they get sick or bored of a story, they have no reason not to stop writing.
Subverted, for the most part, by Gender Confusion. The author originally updated irregularly, having only intended for the story to be a humorous oneshot. She now updates at least once a week, and for the anniversary of the first chapter, took it as a self-imposed challenge to write a chapter a day. The author is well known for keeping strict schedules for all her major works, only breaking them for major reasons (the only one for this work was a two-week pause during which the author was suffering medical issues and spending some fifteen hours a day at school), and always warning the readers ahead of time when she needs to take a break for whatever reason.