This is more of a teaching article, but it contains a good explanation for some pronunciation issues and some suggestions for working on those issues.In short the author indicates that there are 3 reasons why language learners have difficulty with some sounds.
1) The sound may not exist in their first language.
2) The sound may exist, but it is used differently in their first language.
3) The sound may exist in their first language, but it is not perceived by them. Dan