This is my standardized procedure in inviting someone to Signal Messenger to minimize risk of failures.

1. Check if your invitee uses free data or zero-rating internet. If the invitee does not have a stable mobile data, you will have a harder time contacting them on Signal.
2. Check if your invitee’s Android OS is killing the app, refer them to Don’t kill my app! or if you have physical access to invitee’s phone, check their settings to prevent the Android OS from killing the app like disabling battery optimization for the Signal app.
3. Check if your invitee’s Android OS is preventing the notification of Signal from appearing. If you have physical access to invitee’s phone, check their settings to prevent the Android OS from blocking the notification like notification filter or enabling the notification for the Signal app.
4. If your invitee’s phone is running Android 11, try to enable the native chat bubble feature of the Android 11.
5. If your invitee is technically minded, convince them in turning on backups in Signal just in case.
6. Make sure your invitee is committed in chatting you in Signal for the sake of privacy, and for chatting you, or else you will just waste time inviting your friend if they are not committed in chatting you.