John Hopkins Magazine has an interesting article on sounds and why it can be so difficult for some learners to produce a sound in the same way a native speaker can.
Essentially it says that when we are young we pick up a set of sounds from the language we hear around us and store them in our brain. However, later in life we might hear new sounds from a foreign language and although our ears pick up the different sounds when they are processed by the brain they are not added to the bank of sounds already there but simply paired up with the closest match.
So as teachers we need to explicitly demonstrate to our students new sounds make sure they can identify them (minimal pairs is an obvious route here) and then when it comes to production show them the precise way the mouth & tongue should be formed to produce the sound properly.
Take a look at the article to read more on this.[ratings]