There are two reasons for this issue:
1. Inbox is overflown.
Inbox can store up to 400 items. The game will delete the oldest item in the Inbox if there is no more space for a new one.
2. Items have expired.
Each Inbox item has its expiration date, depending on its origin.
For example:
- 100 days for Shop items, Special Offers, over-stacked Champions, and Artifacts
- 7 days for Event and Tournament rewards
- 1 day for various quest rewards
Items will disappear from the Inbox upon expiration. Remember to collect them in time!