Players constantly declining

I recently started a save with Manchester United. I immediately put my focus on giving Axel Tuanzebe game time (he played 49 games in all competitions in the first season) and yet he has quite rapidly declined as a player. It is an identical situation with Aaron Wan-Bissaka and Dean Henderson, who my staff claim to be 'progressing well in training'.

I can't find a solution. I have upgraded all of my coaching staff, I have assigned mentoring groups, I have upgraded my facilities to state of the art, I have given plenty of game time, I have tried to take over individual training... I am only just starting my second season but nothing can stop these players from their landslide and it's putting me off continuing the save.

Any suggestions?