My team just gave up on Slack today, because push notifications seem too unreliable when we're on our phones. Paradoxically, we've found it works better when the app is not running than it does when it is running. Most of us are using Android phones. I couldn't find anyone else having the same problem, and I couldn't find any answers googling around, other than the obvious "turn on push notifications"-- push notices were definitely on.

We may go back to Slack at some point, if we can get it to work effectively for us.