How can I ask about a person's specialty? I'm learning Japanese from a book, and it says that "specialty" is []{}, but Japanese speakers can't seem to understand me if I use this word. How can I correctly write ("What's your specialty?") in Japanese?

is a perfectly natural way of saying "What's your specialty?", but this question would make sense only to professional scholars, physicians and such. You would expect answers like "algebraic topology", "plasma cosmology", "pediatric hematology", etc.

To ask one's major to college students, works, but it's better to ask , because usually refers to true experts. To ask about someone's job in general, see the answer of @IgorSkochinsky.