I've been unable to unlock Maybel as a challenger despite buying many minions (far more than five) directly from her store and deleting them as per the instructions. I've tried multiple times over the past few weeks and it hasn't worked. There are other users over on the Battle forum who have had the same issue so I think it may be glitched.

Ok perfect, thank you!

In case it helps determine the cause - I'm also having an issue :) unlocking Maybel.

Same here, deleted tons of minions purchased from Minions Market last night and no unlock.

I tried to unlock her too but it didn't work. I hope his can be fixed :)

I just tried it again and it worked! Thank you :)