I had some cases where a partymember didn't jump, and it seemed to occur more often when I had a bigger party (removing the party limit with a mod). After I noticed that a member was left behind, I could return and make him do the jump. Nothing prevented them from doing so. I think that the reason is the order in which the game makes everyone jump. It looks like (though I have no solid proof) if there's someone in front of the one that the game wants to jump. He skips it. Then the other party members jump, but the one that was skipped is left behind.
