Also, ffrog is a feminine noun, and gwen is the feminine form of the adjective?
Why is gwyn wrong?
Adjectives soft-mutate after feminine nouns, and g soft-mutates by dropping.
FYI, I had the exact wording as the answer but it was counted wrong.
Unless you say what kind of question it was and exactly what your complete answer was, there is no way to tell what the problem might have been.