Skylanders ship in waves. This means that not all new Skylanders characters are available on the same day the game is released. This also means that as you get further away from the release date, many of the early releases become hard to find and some even go out of production.
Generally there are 5 Waves per game. These Waves are not officially announced by Activision or the stores. So it is often hard to identify which Skylanders characters belong to which Wave and when each Wave actually begins.
The shipping containers that stores receive are marked with a Wave number, but these are usually removed before they reach the sales floor and therefore are not usually seen by the general public. To make things even more confusing, Activision has been known to sneak in extra Waves (i.e. Wave 2.5, Wave 3.5, or Wave 5.1).
Due to the unpredictability of these Waves, fans and collectors always need to be on the lookout for new characters when visiting their local stores.
If you've ever worked in retail, you probably have a pretty good understanding of why Skylanders ship in waves.
It takes many, many months to push out the millions of Skylanders produced each year. So by creating Waves they can prioritize which figures need to be produced first and ship them to stores while they get started on the next Wave.
Product creators and vendors typically have to pay to get shelf space in stores. Naturally, the more shelf space you want, the more it's going to cost. But at the same time you can't fit 30, 40, or 50 different products in a 4-foot space. So unless you're Lego (who will buy out entire aisles in the toy store), space is limited.
All in all though, I really don't think Activision really minds this arrangement. As a byproduct of the limited shelf space and production waves, it means that they are putting new product in the stores almost year-round. This means that they have fans and collectors like us in the stores looking at and talking about their product all year long. I don't think they could ask for much better than that.
While it is fairly common knowledge that Activision does not publicly announce their release dates, data collected over the past five years shows us some semblance of a pattern.
Wave 1 is released on Day 1. This always includes the Starter Packs as well as a large portion of the figures. So in a way they do announce the release date for Wave 1, but that's only because it coincides with the release of the actual game. Even though we have the date for this Wave, Activision doesn't officially announce which figures will be in this Wave. In any case, we are always able to acquire this information prior to Day 1 thanks to the retailers. The games typically release in late September or early October.
Wave 2 has always been a bit of an enigma. The full release of Wave 2 is usually about a month after Wave 1. However, since Toys R Us usually gets an Early Release Date, we often end up seeing Wave 2 the same week as Wave 1. Or in some cases (when store employees get confused) we see Wave 2 on Day 1 as well. Ultimately though it will take about 2-4 weeks before you start to see Wave 2 regularly in all stores.
Wave 3 is a bit unique in that it always comes as a Split Release. This simply means that we see about half of the Wave before Christmas and the remainder of the Wave shortly after Christmas. Toys R Us will once again have and Early Release Date here so it usually starts trickling into their stores in early December (although depending on the release date of Wave 1, this could be a little bit earlier or a bit later). While Toys R Us gets them in plenty of time for the holidays, other retailers will probably only have them for about a week before Christmas.
By the time Wave 4 rolls around there aren't many figures left to release and folks start to wonder why they still have Wave 5. However, it is around this time that they will start to replenish Waves 1 and 2 for those who are new to the game. So between the new Wave 4 figures and the replenishment stock they fill up the cases and the shelves once again. Wave 4 typically starts hitting shelves sometime in March.
Wave 5 is barely a Wave. There is usually only 1 or 2 standard figures left to be released. The rest of the Wave is fleshed out by replenishment and Single Packs of figures that had only been available in Double or Triple Packs in an earlier Wave. Wave 5 typically pops up in May although we may see a few variants (not really part of any Wave) show up as we head into the summer months.