Please make sure you're using the corresponding Ad ID..
For example: There are three types on Admob (and on C3 Advert plugin)
1) 'Interstitial'
2) 'Rewarded'
3) 'Rewarded Interstitial'
..and if you use a 'Rewarded' Ad-unit ID on a C3 action of a 'Rewarded Interstitial', then it won't show.
Also, try Creating it first (ideally on the menu/loader layout with Show: False), then 'Show' it when needed, so that it has enough time to Load and be ready when needed.
EDIT: However, in your case, I know what's the problem, see below: