There seems to be a bug or design overlook in party finder.
When you, for example, decide to recruit 1 Healer and 1 WHM (WHM or SCH), and WHM joins your party, the WHM gets assigned the Healer slot, leaving 1 WHM slot open, thus preventing SCH from joining, even tho, at the time of setting up the party finder the SCH could've joined as Healer and WHM as WHM.
Not the best design.