Supposedly City of Ashes was going to begin filming a few months after the CoB movie, but the script for CoA (written by the same screenplay writer as CoB) was WAY different than the book (even Cassie had mentioned that the script was extremely different than the book.
From the moment I finished watching CoB I knew they were going to have to add A LOT of changes to CoA just to cover for what they did in CoB in order to keep CoA somewhat recognizable to the source material (these changes in addition to all the ones normally made in adapting a work into a film or show). A bit unfair to blame the director (as the CoB director won't be directing CoA) considering that what's her name (the Script Writer) didn't really have any movie experience in writing scripts before CoB (and the movie was FAR from a great adaptation), and considering her the screenplay for Screen Gem's new Tarzan movie (a modern version) was completely panned by critics and Rotten Tomatoes (So I don't know how the person landed the position of being the script writer, the least they could have done was have someone else more experienced as well).
Now to answer you're question as far as I know, (Screen Gems?) CoA is still in pre-production due to the massive changes done in CoB, and in the original CoA script that screwed with the overall plotline of the first 3 books. I'm sure CoA will be developed, it's script just needs to be fixed, along with their advertising campaign. And since they'll need fans of the books, they're probably trying to make the adaptation follow the book's plot as faithfully as possible and not have the movie be an adaptation in name only. The actors don't know much either.