Hi @cmolarte,
Can you be more specific which participant has the issue and what is the issue you are referring to? Both the Android and the iOS app collect the data and upload them automatically, but they do require to remain operational in the background in order to do so.
For the apps to remain operational when you close them, the participant should provide the permissions that the app requests. The permissions vary by the study and depends on the data you intent to collect, but most often includes the permission to send notifications and the permission to access GPS (even if no GPS is collected, as discussed here ).
The articles in this page can also help on what has to be done on Android and iOS to ensure the app functions properly in the background.
If there is a specific case you have in mind, please let me know.
Bests,
Mohammad