This article explains why you might not receive your Arena / Grand Arena reward.
Didn't Get the Daily 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 clicking your avatar in the upper-left corner. Note that successful defenses are not taken into account when issuing the reward.
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. Thus, each day you can receive up to 50 Arena Coins and 50 Grand Arena Coins for successful defenses.