Firing number two

I have rebuilt this kiln from an empty shell and a pile of bricks. I did the first firing in it last week but it didn't get up to temperature, I was hoping to get it to around 1260-1280 degrees C (cone 8 or 9), but only reached 1170. The back flue wall had shrunk away from the sides during the firing which may explain the failure to reach temperature. The gaps have been plugged with ceramic fibre and the second firing is underway.