I don't ever recall seeing it in the anime. They just take the ass kicking shake your hand and wait for the character to leave.

But my thought usually is FULL RESTORE SPAM. Or that they do one challenge a day, or one fight during appointed times (As trainers are actually pitted up against each other before they get to the leader).

It seems unreasonable, but there aren't a lot of trainers that appear to even take the challenge to begin with, they get weeded out. And a lot don't even seem to make it to the elite 4.