I entered the A400 code on April 9. The suggestion above sounded good because my travel has picked up unpredicatably, such that a challenge starting April 16 would be good.
So I called CS to see if entering the A400 code created a challenge
It did not.
Perhaps one has to become "targeted" for it by calling to ask for a challenge.

edit: based on discussion in a related thread, I called CS again and this time they not only activated the plat challenge, they also retroactively included flights taken 4-16 through 4-30, even though they already had posted to my AAdvantage account. The CSR's reasoning was that I had done something April 9 that I reasonably believed would establish a challenge. This appears to be an exception to the well-documented rule that posted flights won't count on a retroactive challenge (search for retroactive in the sticky thread). As the CSR put it, his supervisor allowed him to make the challenge not be retroactive.
PS: This retroactive deal was the CSRs idea, not mine -- I'm not that clever. Fortunately, when he suggested it I was too stunned to respond with more than 'huh? wha?' to which he said, 'yeah, can you hold a few minutes?'
When at first you don't succeed, call call again.