The TV will switch to the corresponding EXT input if pin 8 of the scart cable has contact and active (high), ie. vcr switched to play or menus, video + etc.
It may be other pins (video, audio or both) of the scart are faulty or just have poor contact. Very common is either sound with no picture, or picture only.
Some scart cables have very thin pin contacts and result in unreliable connection. Use a quality scart lead with, quality 'thicker' pins (expect to pay upwards of £20 for a good lead). Also, it could be a combination of slack pin contacts on the TV socket and poor 'thin' pin quality on the scart.
A less likely cause is dry soldered joints to the TV scart socket.
That is quite rare. As is, internal TV/AV switching faults in the TV itself.
You can now buy scart leads with additional securing grips for a more reliable connection.
Cobweb
January 2005