That's pretty normal. If you have no quests anymore, nor can you do ninja exams, then you have to wait until tomorrow. You can spend stamina doing "sweep" too, each stage will give you 1.5k xp.
When fighting, you get 1500 but your ninjas gain 15000/20000. Shouldn't our main be the ones getting the bigger exp since our ninjas can't level up pass our main level?