You've tried 150 times for that 5x1% chance at a heroic dragon for 100 gems offer?
Did you have 15000 spare gems to use () or did you use the same few gems over and over?
Myself, I have been playing for 2.5 years and am level 100 with 812 dragons. I am largely free to play but am no newbie.
So I assume that many events, like the breeding islands and this offer, are set up like those maze islands with the hidden gem stash, where we all have a different number of squares to open before we get the gems. I think we all each have a different number of of tries assigned to our account at the start of the event, which doesn't change, and some are lucky and have a small number while others have a big number that is too expensive to attain the prize with.
I unfortunately did not get any gems in the freebies, despite spinning 3 times each day. This might also be tied to an assigned number, don't know except I'm an unlucky one.