Hello,
tracker_device does not show on the ADH when a player joins the game
Unreal Editor for Fortnite
Devices
Join a game in progress
Ath display on screen
No ath displayed, but progress still increases
PC
Did you assign the tracker device on game start and assign when joining in progress? If not all players who join later (or even all depending on the first setting) are not assigned to the tracker and should be assigned manually if I’m right. See:
I think you want the options Assign when Joining in Progress should also be set to true:
Assign When Joining in Progress On, Off
Determines whether a player will be assigned this tracker when they join a game already in progress.
If this is indeed on, can you then maybe share the trigger event you try to read and what other devices you use? As we can then try to solve this mystery together 
Kind regards,
Madelyn
“NoPhear” already wrote: “No ath displayed, but progress still increases”, so he has no issue with assigning the Tracker. This is a known problem with Tracker UI for Join In Progress Players that is like this for many months. See for example this thread (or you can find many more about this):
It was supposedly to be fixed in update v33.10, but it was only improved a little bit = Tracker UI is still not reliable and some players in some maps will not see any Tracker UI on the HUD when they join a session in progress no matter what (while their tracking functionality will normally work for them in the background - they will not see info about their progress on the HUD which is really very bad for User Experience if you are using them for something like Challenges or Quests so you really need to have them visible).