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 (retroactively named Part 1) was very slow with uploaded "Nocturnals", sometimes getting a bi-yearly schedule, but garfieldodiesaved it by uploaded the rest of the stories as Part 2, explaining that the delay was because of real-life issues.
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.