I know. My daughter and I love these challenges too. We have a goal to work through every single grade level someday and the mastery challenges help us cover more ground, especially in areas we have mastered. 

But Khan has a great reason for the time that these challenges are not available...

"Mastery Challenges: a set of challenge questions that pull from various skills, requiring a student to demonstrate mastery of a skill after a period of rest from practicing the skill, in order to truly demonstrate mastery; taking Mastery Challenges is the only way for students to achieve the “Mastered” status level for each math skill."

So take a break, sounds like you deserve it. Keep checking back. We love having you here and hope you enjoy it too.