I got my rewards for both, Arena and Gauntlet (#76, haha^^). But I had a row of about 5 weeks where I did not receive the Arena rewards.
Solution for me was to log out from my account, restart the app and log in again. Maybe it would be even better to force quit the app first, clear the cache of your phone and reboot the phone before you do the log-out-log-in thing!
I hope that helps!