Why do you think that mastering the accent is considered the most common language problem encountered by learners?

It is difficult to master an accent until one is fully familiar with that language as it takes time to understand words and pronounciation. The more used to the language accent the more proficient in that language. Listening to an accent by various meand may be useful to understand and speak in that accent. 

Accent is different in all countries around the world. American accent is not like British accent.somemany words are different in spelling also.that makes confusion among learners.that's why it's encountered as the problem of language learners.