The DAs often times have crowded superior court dockets and if the case doesnt get reached during a specific session, it just gets continued. Each county is different but that's most likely what is going on.
If unable to reach a plea agreement the case will typically either be left on the administrative superior court calendar or transferred to the trial calendar, depending on the backlog. At that point it becomes a priority matter (sort of a first come, first serve) in that older cases are typically set for trial first. Even if placed on the trial calendar for a particular week, your case may not be reached depending on the number of cases ahead of yours and the length of those trials. As Adam said, this does vary by county.
Your attorney may be able to bring a speedy trial motion, but that is difficult, complicated, and unlikely to succeed. Nonetheless, you should speak with your lawyer about this issue.