Fishing is shady overall, it's been discussed countless times. At least for big fishing there seems to be an invisible cap in catching rate (15-20%??) so yea maybe you wasted your gil in melding gathering after all. Personally i did feast of famine with ~700 gathering with endoceras,nepto caught from first try. I had a lot of trouble with takitaro-namitaro but not to the point where i got despaired and of course helicoprion with the nope-weather. Other than that, the difficulty was rather expected as i already knew where i was stepping into lol.

It might seem ironic, but as fishing irl needs patience, this is the case here as well. So take it easy and you will get all of them eventually.

P.S: May i ask, how exactly you reached 900 gathering and what is your Perception and GP?