Are you getting notifications for phone calls and other notifications or just not getting them for Text messages? Also I would try another messaging app to see if you having that issue?
Have you confirmed the software on your device is currently up to date?
Let's try wiping the system cache partition to see if this helps any.
- Press and hold the Power key, tap Power off, and then tap OK.
- Press and hold the Power key and Up Volume key at the same time.
- Release the Power key when the Samsung logo appears.
- Release the Volume keys when the Android image appears.
- Press the Volume keys to highlight Wipe cache partition.
- Press the Power key to select.
- Once “Cache wipe complete” appears, press the Power key to select the Reboot system now.
- The device restarts once the wipe is finished.
I would also try what @crrice6640 recommended on trying a third-party messaging app to see if you still experience the same symptoms. You can search for one in the google play store.