When Lucy returns from Narnia after her first visit, her siblings open the wardrobe to confirm her story. However, they find it to be a normal wardrobe.
Why did the wardrobe work the first time, but not the second time?
@TillB is correct, the wardrobe only worked when you didn't expect to anything to be there. It was the same in both the book and the movie.
Here's an excerpt from the book, right at the very end. The bolding is mine, to emphasize what's relevant to the question, but the italics are in the book itself.
And that would have been the very end of the story if it hadn't been that they felt they really must explain to the Professor why four of the coats out of his wardrobe were missing. And the Professor, who was a very remarkable man, didn't tell them not to be silly or not to tell lies, but believed the whole story. "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. And don't mention it to anyone else unless you find that they've had adventures of the same sort themselves. What's that? How will you know? Oh, you'll know all right. Odd things they say - even their looks - will let the secret out. Keep your eyes open. Bless me, what do they teach them at these schools?"
While it is technically Aslan who pulls the children into Narnia whenever people from our world are needed, he does seem to consistently only do it when we don't expect it.
And this isn't really the ramblings of an old man; the Professor is Digory Kirke from The Magician's Nephew - he was in Narnia while Aslan created that world, got there by way of his uncle's magic rings, and was the one that created the wardrobe from the remains of a tree whose seed was from Narnia. He may not know everything about it, but he does have some experience with magic.
Thats not right, look at the facts. Lucy opened the wardrobe once and didn't expect anything to be in it. But another time in the night she opened it again and she knew before what was there from the last time she went to Narnia and when Edmund opened the wardrobe, he had followed Lucy through last time the wardrobe was opened thus he expected something to be there when he opened so from now on I have officially debunked this mythology.