When forming a new companion-led party, the game spawns the new party in a random place within a certain distance of yours. If it spawns too far from the looters you're right next to, the new party may not engage them without further input from you. If you're in a kingdom, and if you gave the companion enough troops to be at 40% of max count, you can put them into an army, chase down the looters again, and dismiss the companion-party from your army. If you're close enough to the looters, your companion should immediately engage.