Another reason sales may drop off is because your gigs get indexed in with the great masses when they’re no longer new. If you notice, one of the search parameters is for “New” gigs, which regular browsers of the site probably check out. Once you become levelled, your gigs are part of the larger offerings with all the millions of other gigs.
With time, as you establish a good reputation, and as long as your offerings are unique, well-presented and aren’t part of a highly crowded category, you’ll probably start seeing more dependable sales. Another thing to keep in mind, however, is that everyone’s business goes up and down, and not at the same time or for the same reasons. (Do a forum search on your question; you’ll see it’s a very common complaint for all levels, not just newcomers).