When you log in to the game, the game checks with Steam which DLC you own. If the game is told by Steam that you don’t own any DLC, but the game detects DLC items in your inventory, we hide them in order to protect and maintain the value of DLC items for those who have purchased them. This happens if:
A: a player has refunded a DLC or
B: steam is in crisis and cannot communicate with the game what DLC you own
In the case of the latter, just waiting for Steam to ‘wake up’ will fix the issue. In the case of the former, well… you can’t own what you don’t own. 