After upgrading from a Samsung S8 to a S10 with Android 9 aka P or Pie, my notifications were delayed by minutes or hours or sometimes a couple of days. And repeat notifications for Textra, which I use for work notifications, are gone now as well. I am posting the things I tried that have been advised on the Internet, but that didn't help:
o Shut off and leave off for 10 minutes then restart. At least initially this seemed to work, at least for Textra, but the repeat notifications on Textra didn’t work.
o Settings > Connections > Data Usage > Data Saver > turn off (mine was already off)
o Turned off adaptive battery function (mine was already off)
o Switched from Power Mode: Optimized to High Performance, didn't help.
o Turned off put unused apps to sleep: Device Care > Battery > three-dot-menu-button > Settings > Put unused apps to sleep > turned off and rebooted. Didn't work.

o Downloaded the Push Notifications Fixer app (resets “heartbeat” which is how often Android syncs with Google servers) and set the heartbeat to every 5 minutes (default). Didn't work either.
I figured that maybe one of the above might help those with delayed notifications or no repeat Textra notifications. And maybe, just maybe, someone will post a fix here for my ongoing problem.