Why do some people have more difficulty than others when learning a language?

I think some people have an ability to learn languages. So they can speak some languages after learning them for a short period. However, some people are suffering to learn a language. Even though they went to a country to learn the country\'s language, they are not confident to speak the language and have a difficulty to speak it. Fortunately they who have difficulty to learn a language might have other abilities except for learning a language. They might be good at doing math or science. Each person has their own abilities. Practice is the best way to be able to do. So I recommand them to make more effort for learning a language than other people who have an ability of language.