Well Ash isn't an idiot anymore so he doesn't have to be bossed around. And now that Croagunk jabs Brock and takes him away, she doesn't have to do that either.They should really bring back Misty. I miss how she boss Ash around when she's angry & drag Brock away from getting turn down by Nurse Joy or Officer Jenny.
Misty only worked when Ash was a moron. Now that he's grown more mature in recent seasons he has no reason to have someone scold him, which is why Brock doesn't teach him anymore either.