The -ka (家) suffix implies a degree of expertise and traditional authorship. For example, this term would not be applied to a writer creating a story which is then handed over to a manga artist for drawing. The Japanese term for such a writer of comics is gensaku-sha.
So basically if you draw up manga for a person who just gave you a story, you are a Manga-ka.
If you are the writer of such story, you are a Gensakusha.