Recall only offers trackers that are active and tracked for that pet, and only for pets who are active. One of these settings is usually the reason a tracker is missing.
Check that it's tracked for this pet
Go to Care › Manage › Trackers and select the tracker. Under Tracked for, choose All pets, or make sure this pet is selected under Certain pets. Select Save.
Check that it isn't archived
Archived trackers are hidden when logging. In Care › Manage › Trackers, open the Archived section and select Restore.
Check that the pet is active
Pets who are archived or have passed away aren't listed in Log something now. For a pet who has passed, use Add to history on their profile.
Search for it
In Log something now, type the tracker's name in the search box. The list is in the order set in Care › Manage › Trackers.