Android notifications send but not all users receive them

Hello all,

I have some customers who don't receive notifications from my app.

Most of them do receive them.

Some of them only receive notifications when the app is open.

Some of them don't receive at all.

Google developer support said that there is nothing wrong on my app side (server side) because most of the users (about 90%) do receive all notifications.

So I have no idea what more can I do.

Thank you