This was reported by Milecards a little while ago, but I haven’t seen any coverage on other blogs. It looks like there is a bug with American Airlines’ homepage whereby if you start an award search there, you will potentially see less availability than if you started from the Advanced Search page. This is particularly common if you are looking for Alaska availability.
In the image at the top of this post, the search on the left was started from the homepage, while the search on the right was started from the Advanced Search page. As you can see, there is no MileSAAver availability on the left, while there are six dates with availability on the right.
Long story short, start all of your American Airlines award searches from the Advanced Search page.