The store I worked at when I was a student went out of business. It was very popular.
Why? Was it because of a lack of staff?

Question

Answer

I agree with Paul. In the original version you ask WHY right after saying it was very popular. So it seems you are asking why was it very popular, in which case, 'lack of staff'. makes no sense.

Answer

The store I worked at when I was a student went out of business.  It was very popular.
Why did it go out of business?  Was it because of a lack of staff?

*Very good.  I understood the meaning.  I fixed only the "Why?", in order to make it clearer.   じゃ また.

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