Thank you for the feedback. In case it is indeed the same reason which applies to both our cases, I wonder what traffic threshold for a channel or URL may be necessary to trigger appearance in the placement tool.
One of my sites and Custom Channels incurred a bit over 1000 Page Views according to the AdSense stats in the last 7 days. I presume that is also the number of ad-impressions. Could that perhaps be a too low figure for it to appear in the managed placements in comparison to any of your examples?
Maybe Google AdSense also consider other ranking factors that triggers a show or no-show amongst possible managed placements.