How did Lucy/Edmund travel multiple times to Narnia (in The Lion, the Witch and the Wardrobe) when it says later you can't travel the same way twice and certainly not if you’re looking for it as Lucy found it the first time then went back via the same wardrobe to see Mr. Tumnus before Edmund came and found Narnia?
It's important to note that in each incident, there was at least one person involved who did not expect to travel to Narnia:
- Lucy hides in the wardrobe and is unexpectedly transported to Narnia
- Lucy enters the wardrobe, expecting to go to Narnia. Nobody else believes her at this point, so Edmund follows her to have a laugh at her expense, but finds himself in Narnia, where he meets the White Witch
- All four children hide in the wardrobe. Peter and Susan did not expect to go to Narnia
Remember, Professor Kirke had also been to Narnia, and also as a child. Now, he was an old man, and had undoubtedly tried many things to go back himself. Hence what he tells the children at the end:
"No," he said, "I don't think it will be any good trying to go back through the wardrobe door to get the coats. You won't get into Narnia again by that route. Nor would the coats be much use by now if you did! Eh? What's that? Yes, of course you'll get back to Narnia again someday. Once a King in Narnia, always a King in Narnia. But don't go trying to use the same route twice. Indeed, don't try to get there at all. It'll happen when you're not looking for it."
In each subsequent transport incident, nobody was trying to go to Narnia.
- In Prince Caspian they're literally yanked off a railway platform
- In Voyage of the Dawn Treader they're sucked into a painting
- In The Silver Chair, Eustace invokes Aslan's name, but unexpectedly goes through a gate into Aslan's country.
- In The Magician's Nephew, Digory, to escape Jadis, jumps into a random pool in the Wood Between Worlds
I'm not counting the end of The Last Battle since that wasn't Narnia they wound up in.
It's clear that what is meant is that a method of traveling to Narnia will only work for one incident. The whole episode with the wardrobe counts as a single travel incident in this accounting.
Note that the travel in the first book works in a rather different way from subsequently. The children, having grown to adulthood, are de-aged without explanation. (The BBC television series adaptation had some nice acting by one of the boy actors as they stumble back out of the wardrobe, feeling for his beard that had suddenly vanished.) Not having intended to write a sequel, Lewis had not thought about making the dimensional travel consistent at that point.