Doe makes a great point. To add on that... We have had a few "dominant" horses which lack confidence and or leadership skills. The herd will stay clear of their heels and teeth...( so people only watching that part will say they "respect" them, which isn't entirely true) But other than that they will not follow these horses unless there is a "crisis".
Now, I have a very low ranked little mare with brains, speed and a natural curiosity which just beats anything I've ever seen. She is like the horse Doe is talking about... Everyone picks on her... Until they need her... Then she leads the way to safety. Not at all a dominant horse, but with the other skills needed to lead.
On the other hand, another observation I've come across is the 5-6 year "phenomenon". I've found during these two years there is a shift that occurs in many horse's temperaments. For better or worse. I have a young gelding who was bold as brass until he hit 5 years old... He'd never spooked while riding him until that point, now he's like sitting on a jackrabbit out on the trails.
Another mare went the other way... Always nervous and high stung... Suddenly it was like she had an epiphany and decided it was all too much energy being wasted and she became totally solid... Overnight. Bizarre.
I have no idea why, exactly it is... Nothing with either of these horses had changed, from what I could tell... It was almost like as they reached the age of maturity their minds just settled one way or the other.
The other side of this is... I have Only very rarely met a horse who isn't kicking out of some feeling of defensiveness. So I'd start looking for what might be making her feel insecure. Is she feeling too much pressure from you when you ask her to lead? ( it's a big responsibility being up there... Some horse's just don't like that, pushing them can make the feeling worse). Has her tack been checked to make sure she's comfortable? (depending on the horse, being uncomfortable physically can be something they can handle until they feel stressed... Then they need to do SOMETHING). Just see if you can find anything that is new or different, at all, even if it seems too small.