PopIn sends Android notifications as high priority, so they normally arrive even when the app is closed. Some phone makers ship battery savers that hold back or drop notifications from apps they've decided to put to sleep. It's not a PopIn bug: it happens to every app on those phones, WhatsApp included. A couple of settings sort it out.
Which phones
Mostly Samsung, Xiaomi (MIUI / HyperOS), OnePlus, OPPO, realme and Huawei. Pixel and most other phones are fine out of the box.
Let PopIn run in the background
Open your phone's Settings → Apps → PopIn → Battery and pick Unrestricted (some phones call it "Don't optimise" or "No restrictions"). The exact menu names shift a little between phone makers.
Xiaomi and OPPO: turn on Autostart
These phones have a separate Autostart switch. Find PopIn under Settings → Apps → PopIn (or the Security app's permissions page) and switch Autostart on.
Check PopIn's own settings
Make sure push is on for the things you care about in Settings → Notifications, and that quiet hours aren't covering the time you expected a ping. See Quiet hours.
Samsung can quietly move apps you haven't opened in a while into "Sleeping apps". If notifications stop again after a few weeks, check Settings → Battery → Background usage limits and take PopIn out of the sleeping lists.
Still nothing?
Open PopIn once so it can re-register your phone for notifications, then try again. If they're still missing, get in touch and tell us your phone model.