Did you maybe replace the bow with something higher level? Class changes are handled entirely tied to the equipped weapon, and you need a weapon low enough for BRD50 to equip. Equipping a weapon that is job-only will simultaneously equip the Soul stone. If the weapon is class|job then you have to equip the weapon then the soul stone.

Remember to save a new gear set every time you unlock and equip a new job, so that you can fast swap via gearset.