...Well, do they want to?
Not all players wanna lead. Sometimes a perfect balance for a group of 4 isn't 25/25/25/25, but 40/30/30/10.
You should prepare for situations that one character in particular will shine, but also ready for times when they don't want to. (There are people who actually like to play support and ride along through a story. Though that is completely different from someone you can just ignore and treat like luggage.)
So is it, right this current moment, a problem that is troubling your PLAYERS?
Sometimes go OOC and ask them directly might be the best answer.