How early did you submit your panel?
To give you an idea of how early you need to act, here is the timeline from my personal experience.
January 30: I submitted my panel and Fanime acknowledged.
February 16: Fanime contacted me to confirm my time slot requests.
April 11: Fanime proposed satisfactory time slots which I gladly accepted.
Another issue you may have is that someone else may also be running a panel on a similar subject.
To give you an idea of how early you need to act, here is the timeline from my personal experience.
January 30: I submitted my panel and Fanime acknowledged.
February 16: Fanime contacted me to confirm my time slot requests.
April 11: Fanime proposed satisfactory time slots which I gladly accepted.
Another issue you may have is that someone else may also be running a panel on a similar subject.