Have you accidentally used a banner ID as the interstitial ID or vice versa?
Check the ad ID in C2 for leading or trailing spaces - I've had a banner not show because of an extra space in the ID!
You should still see test ads even on android v4.0.4. For some reason however, I always see the interstitial test ad, but not the always banner test ad. When it goes live, they both show OK.
I only preload once at the start of the game layout, and just use show/hide during the game. Works fine.