When you purchase an item from the Nexus, the item goes into your inventory. If the inventory is full, it goes into your backpack (if you have one equipped). If they're both full, it is sent to your gift chest.
First and foremost, make sure to check your inventory, backpack and gift chest thoroughly for what you've purchased. If it is not there, restart the game and check again. Sometimes a relog is needed for the purchase to register.
If you still cannot see the item please contact our support team. We will be able to verify your purchase and reimburse you if appropriate.
If you have any evidence of the issue occurring (screenshots/videos) please make sure to include them in your ticket. This is not mandatory, but it will be helpful to us.