Why does fish broth made with frozen fish lack flavour?
I think it's not so much the fact that you're using frozen fish (which certainly isn't the best), but the fact that you've missed an important step: pan-frying the fish before making the soup. I can say that all fish soups I've had have always had the fish pan-fried before boiling, with ginger added, otherwise you just don't get the same flavour.

Another thing is, in a properly made soup, the fish should not have much fish flavour remaining as it should all be in the soup. 

I would recommend retrying the soup with the same type of fish, but pan frying them and adding ginger. If the soup still does not taste great, it could be that the YFG you're buying just isn't very good, and you could consider using a different type of fish.

Here's a link I found supporting my assertion, with more detailed instructions:
http://www.chinesesouppot.com/4-soup-techniques/1195-how-to-make-milky-white-creamy-fish-soup