In the case of book 1 and Flitwick's Charms class - they are essentially learning a new language - a language with an emphasis on tone and inflection - much like Mandarin Chinese.
For example - from book 1:
"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." [Flitwick]
'You're saying it wrong', Harry heard Hermione snap. 'It's
Wing-gar-dium Levi-o-sa, make the 'gar' nice and long'
Consider the word 'ma' in Mandarin:
By saying the two letters 'ma' together with varying inflection, you get 5 different meanings:
mā Mother - High and level - 1st tone
má To bother - Rising from middle to high - 2nd tone
mǎ Horse - First falling then rising - 3rd tone
mà To scold - Falling from high - 4th tone
ma An interrogative particle - Brief and soft - Neutral
source - http://www.learnnc.org/lp/editions/mandarin1/4480
To anyone unfamiliar with the tonal language, somebody saying 'ma' with any of the 5 inflections may not notice the difference and just hear 'ma' but to somebody with an understanding of the language, there are distinct differences in meaning.
The same principle can clearly be applied to charms work given both Hermione and Flitwick's statements. It isn't enough to just say the words, you have to say them correctly. Factor in regional dialects (Seamus Finnigan comes to mind ;), possible speech or physical defects (tongue-tied, dental issues, etc...) and one can suppose that 11 year-olds may not get things perfectly the first or second time