Undeliverable - Android Device

[Ulgrif]

hi,
i have a problem recently on my android user. 80% of my user get undeliverable and dont get the notification at all... they get the notification on the app but my usage is time critical...
i need help!
one user is android 15 on an samsung s24
he has the notification authorised
battery is not restrained
mobile data is background availlable
he has activated the override for not disturb

the notification has work only once on his cellphone... he has reinstall the app last week.
is there a way to know if the problem is from the phone or google?
i guess the problem is an update from the phone since its not all of the user that have the problem...

really need this to work
Thanks Ulgrif

[admin]

Please let me know what kind of devices not work. (manufacturer, model, Android version)

Best Regards
Kevin