There’s no problem in understanding the phrase: after all, it sounds like what we say in English. But it's not actually what we write. We write (and think that we say) I want to ask you a favour.
It's easy to see how spoken you a can come to be reinterpreted as your. From strong juː eɪ we derive weak ju ə, readily compressed to one syllable as jʊə. And jʊə is how some British people (25% of them in my preference poll) pronounce your.
Those who, like me, pronounce your as jɔː, or weaken it to jə, would leave no room for confusion with you a.
— — —
There’s a job going for a Pronunciation Linguist at the BBC Pronunciation Unit. Anyone interested? Details here.
The job demands, inter alia,
• A thorough and practical knowledge of the IPA and its application in phonemic and phonetic transcription of all languages (not just English).
• A university degree in (a) linguistics or phonetics, or (b) modern languages or English with a significant linguistics component, including phonetics.
• Near-native fluency in at least one language besides English, and preferably more than one.