miss fire

I recently purchased a NIB (never fired) 4 inch Colt Python through GB. Yesterday I fired it for the first time. I would like to keep this gun f-o-r-e-v-e-r, and expect to fire it often and maintain it well. So after a mix of 25 - 357 FMJ Magnums and 40 - 38 Special FMJ rounds I had a miss...