Board Thread:News and Announcements/@comment-129.143.71.38-20140131110003/@comment-24952077-20140517100546

After doing some research I think this is the most plausible situation

They need for material to make Season 2, Season 1 covers Acts (Chapters) 1 to 17 in the manga and they are only up to Act 31 (30 being the most recent which was a year ago in 2013) so they need like 7 more Acts of material to make Season 2.

The illistrator of the manga series (not sure if involved with anime) has another series call Triage X which is his current focus, without him they cannot continue the manga the way it is. So until the illistrator returns to working on HTOD (the manga) Season 2 won't exist, my advice would be try and get into Triage X and keep your manga habits occupied until HTOD returns. Just so you know his planning to continue working on Triage X for the rest of 2014 so 2015 is the earliest we will see HOTD, evidence of this is in the link below (look at all volumes and their release dates).

http://www.yenpress.com/triage-x/

Another link showing how I got to this conclusion

HTOD Act 30 Release Date - http://www.yenpress.com/2013/04/highschool-of-the-dead-act-30-now-available/