I came across it in this article.
Chloe Foster, a clinical psychologist at the Centre for Anxiety Disorders and Trauma in London, says shyness in itself is quite common and normal and doesn’t cause problems unless it develops into more of a social anxiety.
Why is there an indefinite article before social anxiety? Anxiety is an uncountable noun, and I had always seen social anxiety as a mental disorder being used as uncountable. Does these all mean that after the phrase more of we need to use the indefinite article before uncountable nouns?