Hi @SmashAndCrab,
Every campaign mission has icons in the bottom left of the screen indicating the difficulty (Easy, Normal, Hard) that you played on that selected campaign mission.
I suggest you to take a look at each campaign mission and make sure to complete the remaining campaign missions on normal difficulty.
Earlier today I checked every mission to see if I missed any missions on Normal. Every mission has been completed on all difficulties. As a matter of fact, I completed a Normal difficulty playthrough of every single mission in every war story today (in an attempt to see if that would unlock the trophy). No such luck.
That's really odd.
I recommend you to backup your Battlefield 1 save data to USB just in case, then delete this save data from your PS4. This will reset your progress in singleplayer and also reset all settings (Your multiplayer progress will not be deleted, it's stored in EA servers). After that, start the game and play the campaign on normal difficulty.
Hope this helps.
