Good morning. Was the squib the first round you fired or the last?
That much unburned powder would indicate a bullet which jumped its crimp. Have you checked your bullet diameter? It should be .452"
Then there is the possibility you are crimping too much. If there is too much crimp applied the case will expand causing the bullet to be loose in the case despite the nice roll crimp you have applied.

Also consider only belling the case enough to insert the bullet.
I suggest you check the outter diameter of the loaded rounds and give us a measurement and I will check some of my own.

This is an unusual problem for the .45 Colt so more info would be helpful.

Bruce

FWIW I find better shot to shot consistency when I tip the gun back after loading. This places the powder back against the primer. Unique in that big case is somewhat position sensitive.