I didn't get the Arena reward at 8 PM
To receive the daily reward for your Arena ranking, you need to attack at least once in Arena or Grand Arena before 8:00 PM your in-game time zone. You can check the time zone by tapping your avatar in the upper-left corner. Note that successful defenses are not taken into account when issuing the reward.
I didn't get Arena or Grand Arena Coins for a successful defense
You receive 10 Arena Coins only for the first five successful defenses, up to 50 Coins per day. All further defenses are not rewarded. The Coins arrive in a letter in your in-game mailbox. The same rule works for Grand Arena.