Chris,
there can be a lot of reasons why it doesn't work.
first of all when you enable it in admob it can take up to a day before it will actually work.
is mediation correctly set up on both sides?
maybe there's no rewarded videos for you (mainly your country) at that time?
good thing is that admob also spits out rewarded videos for a while now.
side node about inmobi...
I think they're mainly India only and I don't think their mediation is still working when I look at rewarded video stat in my console (see below).
That the impressions are so low is normal since it preloads before it enables the rewarded ad button and not everyone uses the button to get a hint in a game.
But 0 impressions on 2629 request is kind of suspecious
(I'll remove them again on next updates)