Article "a" with uncountable nouns

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.

Answer

There is a rule in Michael Swan's "Practical English Usage" (section 149.4):

With certain uncountable nouns – especially nouns referring to human emotions and mental activity – we often use a/an when we are limiting their meaning in some way (and we generally limit it by putting adjectives).

• We need a secretary with a first-class knowledge of German (NOT...with first class knowledge of German ).


• She has always had a deep distrust of strangers.


• That child shows a surprising understanding of adult behaviour.


• My parents wanted me to have a good education (NOT .... to have good education).



• You've been a great help.


• I need a good sleep.

Note that these nouns cannot normally be used in the plural, and that most uncountable nouns cannot be used with a/an at all, even when they have an adjective.

• My father enjoys very good health. (NOT ... a very good health.)


• We are having terrible weather. (NOT... a terrible weather.)


• He speaks excellent English. (NOT ... an excellent English.)


• It's interesting work (NOT ... an interesting work.)

Unfortunately this book does't show a list of such uncountable nouns which can/cannot be used with "a". But the Ngram Viewer shows that "fluency" is not used with "a". See the results of searching:

So, it would be better to say "She showed great fluency in English".