**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).