1. Syntax structure.
These are the obvious reasons for the problems experienced in second language acquisition, and most of them are related that people attempt to learn another language during their teenage or adult years, in a few hours each week of school time, and they have a lot of other things to take care of, instead a child learns via the constant interaction that he or she experiences, and has not many things else to do. Besides the adult or teenage people have an already known language available for most of their daily communicative requirements.
There are other reasons, for example the suggestion that adults tongues get stiff from pronouncing one type of language and just cannot cope with the sounds of another language. However there is not physical evidence to support this.
Maybe the primary difficulty for most people can be captured in terms of a distinction between acquisition and learning.
The term acquisition refers to the gradual development of ability in a language by using it naturally in communicative situations. Instead the term learning applies to the conscious process of accumulating knowledge of the vocabulary and grammar of a language.
Activities related with learning have traditionally been used in language teaching in schools, and if they are successful tend to result in knowledge about the language studied.
Activities related with acquisition are those experienced by the young child and by those who pick up another language from long periods spent in social interaction, the language used daily, in another country.
Those whose second language experience is primarily a learning one tend not to develop the proficiency of those who have had an acquiring experience.
However, even in ideal acquisition situations, very few adults seem to reach native like proficiency in using a second language. There are suggestions that some features, for example vocabulary or grammar, of a second language are easier to acquire than other, for example phonology. Sometimes this is taken as evidence that after the critical period has passed, around puberty, it becomes very difficult to acquire another language fully. It has been demonstrated that students in their early teens are quicker and more effective second language learners than, for example 7 year olds. It may be, of course, that the acquisition of a second language requires a combination of factors. The optimum age may be during the years 11-16 when the flexibility of the language acquisition faculty has not been completely lost, and the maturation of cognitive skills allows more effective working out of the regular features of the second language encountered.
Yet during this optimum age, there may exist an acquisition barrier of quite a different sort. Teenagers are typically much more self conscious than young children. If there is a strong element of unwillingness or embarrassment in attempting to produce the different sounds of other languages, then it may override whatever physical and cognitive abilities there are. If this self-consciousness is combined with a lack of empathy with the foreign culture, then the subtle effects of not wanting to sound like a Russian or an American may strongly inhibit the acquisition process.