Probably a lot of possible reasons for this, without knowing more info. When I was experiencing a similar confusion, I realized that when sending to everyone, they worked. But sending to specified users didn’t. Turned out that that was because the current push notifications don’t support the use of anonymous emails. Are you using real or anonymous emails in your app?
I’m hoping the glide team addresses this soon. Heres where I brought this up and learned that anonymous emails aren’t supported yet. Push Notifications not working for "Send to specific users"