A big part of being the head of a pack - whether you're stepping in or have your own - is to know the members. Know who thinks they're in charge; show them you have taken that role. Know or learn what is needed to lead them; some dogs need different leadership than others. Just like humans in school, not everyone learns the same.
Some dogs like a leader in the truest sense of the word; someone who literally leads, someone they can follow. Others prefer a leader as cattle need a herder.
The largest myth about being the dominant role with dogs is that you must always walk first. This is not true.
Think about it. When a Border Collie herds cattle or ducks, do they do that from the front? No. Are they still leading them where they want to go? Yes.
So when a member of your pack won't follow you where you want them to go, try leading a different way. You're still the dominant role, if you are giving direction.
Know how to lead your pack, however big or small it may be.