Because it occurs to me that it may not be common knowledge - there is a default DND 'do not disturb' setting turned on by slack to mute notifications at night. That's what the little 'z' next to some names in the menu mean. If you have DND on, then notification pop-ups will be muted and you won't get pinged, unless someone speficially tells Slack to override.
If you want to turn this off or edit the settings, click on the little alarm-bell looking icon in the upper left.