Here are the reasons why (the fifth) passive skill does not work.

1.Some heroes' passive skill requires a certain amount of cool time. 

-if the cool time did not pass by, mentioned passive skill will not work. 

2.Passive skill's cool time does not get reduced or initialized from sub(candidate) party. 

-When exchanged with the sub(candidate) during the battle, the hero's cool time does not get reduced.

-When going to the next round, sub(candidate) party hero's skill cool time does not initialize.  

-Only Main(front) party's heros' skill cool time gets reduced and initialized. 

3. Some heroes' passive skills apply only 'start of the battle' from the main(front) party. 

-When combating after exchanging from sub(candidate) party, 'start of the battle' conditional skill will not work.