Someone's not rating you 5 stars, obviously.
http://www.fimfiction.net/story/36641/My-Little-Exalt
The way that ratings are calculated makes it extremely difficult to obtain and keep a 5-star rating, and it gets more difficult the more plays your arc gets. This is simply due to the way people are, and the way numbers work.
If five people play your arc, and four of them give you 5 star ratings, but the last one gives you a 2-star, then you have an average rating of [(2+5+5+5+5)/5] = 4.4, which gets you listed as a 4-star arc, even tough 80% of your ratings were 5-star. If you're trying to get your arc up to 5 stars, every 1-star rating you get effectively wipes out the gains from 7 five-star ratings.
The human aspect is that it's just about impossible to please everyone. Some people want hard mobs to fight, some people want easy ones. Some people like certain kinds of stories, others don't care about the story at all, and won't even read most of the text that you might spend hours writing. People give different ratings for all sorts of reasons, and the sheer weight of numbers forces all the averaged scores toward the middle of the range. The better arcs will trend higher, the poorer ones will trend lower, but most arcs aren't going to dwell in the 5 range for any appreciable amount of time after they've accumulated a significant number of plays.
Edit: And you can't just go by the ratings given to you by people who are commenting on your arcs. Generally, people are more likely to comment on arcs they like. If someone is giving your arc a very low rating, chances are that they won't send you a comment, but they'll bring your average down all the same.
Thank you Geveo for a very well written, and "Answered" question. It now makes a good deal more sense to me then before.
I have 3 arcs ( 9,6,and 3 people have tried them), and all of them stand at 4 stars. Everyone that has left me feedback has told me, they gave me 5 stars.
What does it take to get and keep a 5 star rating, and Im getting so many 5 star feedback but they all stand at 4?