When attacking to colony, I usually get "A defeated D, but was unable to capture the colony" for a few times before they give up. A is attacking unit, D is some unarmed defender.
I'm not really sure what's happening, but I guess this is because D took some auto-muskets from colony stock, lost them but some muskets were left.
The problem is that the dialog really doesn't tell what's happening and this doesn't seem to be documented either. I first thought that there's some reason that artillery cannot be used to capture the colony.
Maybe the "couldn't capture the colony" could just be left off? And if colonist really had the muskets in the battle, it really was a soldier..
Try attacking Isabella in attached savegame.