We're aware of the current issue of the Potent Fine Shadow Crystal Compression Coupon and Potent Finest Shadow Crystal Upgrade Coupons that, when used, do not grant any Nightmare Crystals. This has been fixed as of the 9/7 maintenance.
You can read our forum post about this issue here as well; Issue with Midsummer Box and Magic Academy Suitcase
If you had used one or more of these coupons, Please submit a ticket with your character name and server so we can verify this issue and get you back your coupon and crystals! You will be able to use the coupon for the Nightmare Crystals after our next upcoming maintenance, September 7th, 2017. :)
We do appreciate your understanding and we're sorry for any inconvenience this has caused you.