October 2018
I read in https://answers.ea.com/t5/Game-Information/Challenge-Battles-are-Inconsistent/td-p/7099064 that "Unit levels should trigger Challenge Battles if the enemy's deck is 1.5 levels or more above on average"
In a recent match, my units were:
The other player had units of:
(screenshot attached)
Let's be conservative and assume the other player's units had no training, while bumping up to the next level any of my units that had training. This will underestimate the level difference.
That's a total of at least 10 levels higher across 6 units, but it didn't trigger a Challenge Battle.
Note: my commander was level 7 + 0 training and theirs was level 9 + 2 training.
P.S. I really liked the updates from 1.1 onwards, and am particularly grateful that existing players were given the extra crates to bring everyone up to speed in the new league system.
November 2018
November 2018 - last edited November 2018
Your commander's level also factors into that. Also challenge matches based on levels don't trigger unless you're within 3 of the level cap for the league you're in, which in some of the upper leagues you won't be right now. That said, based on your screenshot you're only 2.5 from the league cap for platinum so I'm surprised one didn't trigger there, looks like a bug, go throw it into the bug report forum.