One thing that went wrong is that he ran out of actors. The typical Hitchcock movie (there are odd exceptions like Psycho) requires stars with a light comedy touch. That sort of actor (Jimmy Stewart, Cary Grant, Michael Redgrave) went out of fashion c. 1960 and Hitch found himself saddled with stars like the uncomprehending Paul Newman, the wooden Frederick Forrest and the inadequate and hopelessly miscast Jon Finch.
The ideal Hitchcock pairing is Cary Grant and Grace Kelly. By the time Hitch came to make The Birds Grant was too old and Kelly had flitted off to become a princess. You can see how Rod Taylor and Tippi Hedren are busting a gut pretending to be Grant and Kelly, but the effortless magic just isn't there.
Tippi Hedren was Hitchcock's attempt to build himself a blonde goddess from scratch. It was an enterprise that ended in ugliness. There were no more muses. What a shame he never got to work with Catherine Deneuve.
Another thing is he got overtaken by the New Wave. There's a story about him going to see something of Antonioni's and coming out of the cinema exclaiming, "Oh my God, these Europeans are twenty years ahead of us!" And so they were. Younger American directors were able to adapt to this but Hitch found it hard. Right up to the end- yes, even in Family Plot- he was falling back on cheesy back projections. One thing I hate about studio productions of Hollywood's "Golden Age" is the reluctance to shoot dialogue or tricky night scenes on location. Our hero and heroine are in Monument Valley. This is established in long shot. Then the camera moves in close and they're standing on a sound stage in front of a backdrop with silly little bushes masking the line where canvas meets floor and the light that was bright and unmanageable has gone all soft. There's an example of this failed sleight of hand in the Birds that makes me want to throw things at the screen.
I've just been watching The Man Who Knew Too Much. Not the 50s remake, but the 1934 original. It belongs to that period in cinema history when the new sound equipment was intimidating the actors and imposing limitations on the camera's freedom to roam and you have to make allowances for this and treat it like the delicate historical artefact it's become. Even so, Hitch's genius as a storyteller shines through. There are lots of lovely little touches, the suspense is gripping and the shoot out at the end (modelled on the real-life siege of Sidney Street) is a tour de force. It takes itself less seriously than the remake. In place of Jimmy Stewart's "ugly American" (was he ever less attractive?) we have Leslie Banks as an upper-class Brit with a touch of the Bertie Woosters and in place of soppy Doris Day we have Edna Best as an Olympic class clay pigeon shooter who- in the final reel- grabs a rifle off a policemen and picks off the bad guy who is menacing her daughter. There are only twenty years between the two versions, but those twenty years have included a world war and the onset of a cold one and everything has undergone a sea change and hardly for the better.