1. Your deck is not yet in the library.
A deck is only displayed in the app when it has been assigned to a topic. A detailed description of how to add a deck to a topic can be found in the article How do I add a deck to a topic?
2. Your deck is set to "not visible".
If your deck appears greyed out in the library, it is marked as "not visible". This hides it in the app. Click on the eye next to the deck name to mark a deck as "visible".
3. Your deck is in a for you not visible content package.
To be able to see a deck in the app, this deck must be in your assigned content package. A detailed description of how to assign a content package to a deck can be found in the article How do I assign a content package to a deck?
4. Your deck is of the type "Initial" and has no cards assigned yet.
A deck of the type "Initial" does not have to contain learn cards and is also displayed if there are only task cards. However, if there are neither learn nor question cards, the deck is not displayed. Add a card to make the deck visible.