TL;DR - the movies are incredibly unreliable.

The process of learning spells is far more drawn out and difficult than the movies indicate. Since the books more or less answer this question I won't do much but provide quotes. Here are some from the first few weeks Harry is at Hogwarts.

And then, once you had managed to find them, there were the classes themselves. There was a lot more to magic, as Harry quickly found out, than waving your wand and saying a few funny words.

They had to study the night skies through their telescopes every Wednesday at midnight and learn the names of different stars and the movements of the planets. Three times a week they went out to the greenhouses behind the castle to study Herbology, with a dumpy little witch called Professor Sprout, where they learned how to take care of all the strange plants and fungi, and found out what they were used for.

In McGonagalls class...

Then she changed her desk into a pig and back again. They were all very impressed and couldn't wait to get started, but soon realized they weren't going to be changing the furniture into animals for a long time. After taking a lot of complicated notes, they were each given a match and started trying to turn it into a needle. By the end of the lesson, only Hermione Granger had made any difference to her match; Professor McGonagall showed the class how it had gone all silver and pointy and gave Hermione a rare smile.

Flitwick and Wingardium Leviosa...

"Now, don't forget that nice wrist movement we've been practicing!" squeaked Professor Flitwick, perched on top of his pile of books as usual. "Swish and flick, remember, swish and flick. And saying the magic words properly is very important, too -- never forget Wizard Baruffio, who said 's' instead of 'f' and found himself on the floor with a buffalo on his chest."

It was very difficult. Harry and Seamus swished and flicked, but the feather they were supposed to be sending skyward just lay on the desktop. Seamus got so impatient that he prodded it with his wand and set fire to it -- Harry had to put it out with his hat.

(snip)

"You're saying it wrong," Harry heard Hermione snap. "It's Wing-gar-dium Levi-o-sa, make the 'gar' nice and long."

In the dueling club in Chamber Of Secrets, Lockhart (attempts) to indicate how to block/parry spells (interestingly we never see how this is actually done, though some of the adults do it on occasion).

He raised his own wand, attempted a complicated sort of wiggling action, and dropped it. Snape smirked as Lockhart quickly picked it up, saying, "Whoops -my wand is a little overexcited -"

Half-Blood Prince takes us through learning to Apparate, which is rather akin to learning to drive. It takes at least several weeks of lessons, and you have to pass a test since it's extremely dangerous. Harry's first attempt...

Harry spun on the spot, lost balance, and nearly fell over. He was not the only one. The whole Hall was suddenly full of staggering people; Neville was flat on his back; Ernie Macmillan, on the other hand, had done a kind of pirouetting leap into his hoop and looked momentarily thrilled, until he caught sight of Dean Thomas roaring with laughter at him.

(snip)

The second attempt was no better than the first. The third was just as bad. Not until the fourth did anything exciting happen. There was a horrible screech of pain and everybody looked around, terrified, to see Susan Bones of Hufflepuff wobbling in her hoop with her left leg still standing five feet away where she had started.

Finally, note that simply getting a spell to work isn't considered a success - you have to master it. Here's a comment about the first year transfiguration exam -

Professor McGonagall watched them turn a mouse into a snuffbox - points were given for how pretty the snuffbox was, but taken away if it had whiskers.

This is a continuous theme - characters casting spells that only half work, or 80% work, and a focus of the teaching seems to be to bring this up to the 100% mark. In other words, it's not just about getting a spell to work, it;s about getting it to work right, in a consistent manner.

I won't include any more quotes, since it's more a matter of stacking little things than any large explanations (gotta keep that magic vague).