I think in order to get the 10th Epic rift you would have had to finish in the top 3 rank rewards of the daily F4 champ use events (at least a couple of times over the month) to get the Master coral to do one more pass, yielding 5k more epic coral.
You get any coral for doing beginner
Have you 100% Normal? You get 10k T1 Coral to then trickle down to Epic Shards if you're half topped up
I just saw that on another thread so i wanted to share it. I hope it helps.