It's a confidence issue as well a match-up issue.
Fed has 3 major weaknesses right now:
1 - He's not clutch enough on the big points, has no bread and butter play to rely on, too much variety is not a good thing against the elite players.
2 - He's not showing enough fight within a match when he's behind, attitude and body language is mostly negative, must display more outward passion and grit.
3 - He can no longer rely on his offense to offset his decline in defense, especially on the forehand side. Must reduce errors and work on more explosiveness on the forehand side.