1. What device are you using?
2. Is the device running iOS 8 or above?
3. If so, try rebooting your device and then go to Settings - Notifications - Messages and then enable the toggle button next to "Show on Lock Screen". Also, enable the button next to "Show Previews".
Now, you do realize that you cannot see the slide to reply option until you slide your finger to the left on an actual message notification.