word choice - How can I ask about a person's specialty?

I'm learning Japanese from a book, and it says that "specialty" is ご[専門]{せんもん}, but Japanese speakers can't seem to understand me if I use this word.

How can I correctly write 「ご専門は何ですか。」 ("What's your specialty?") in Japanese?

Answer

ご専門は何ですか 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, ご専門 works, but it's better to ask 専攻【せんこう】は何ですか, because 専門 usually refers to true experts. To ask about someone's job in general, see the answer of @IgorSkochinsky.