In the last time I noticed that when you sweep ninja exam , when it tells you that is finished sweeping in fact he isn't. It has a delay and if you click to confirm the complete it stops and it's possible to can't finish sweeping. For example today it happend to me :

I can't finish sweeping and now I need to wait till tomorrow to finish sweeping cuz I don't want to change mains for this exam.... again.... (This exam can be done with just lightning main with my current ninjas... and for 288 and higher I need water main and my main team uses earth main so.... idk what to do...).