Transient Thermal Analysis

I am running a transient thermal analysis on solid rocket motor which is initially at 270 deg K. I am applying convection to the outside of the cylinder with a heating curve. The ambient temperature increases from 270 deg K to 1280 deg K in 14 secs and remains constant at tge 1280 deg K for the remaining time.

The model is losing heat somehow and temperatures are decreasing. Has anyone else run into this?

Thanks.