There is such a thing in English that when a mass noun combined with an adjective it takes the indefinite article. For example:
He had a great knowledge of English.
But I have noticed that is not always the case. For example: I have observed that with the word "fluency" and other adjective the is not followed, for instance:
She showed great fluency in English.
Would it be correct to say "She showed a great fluency in English"? Tell me please if there is any additional rule to that.