About your troubles...
I have a theory about the rates of rare bugs and fish: They change from day to day - and they might be based on health luck. For instance: When I got the Saddle Bichir, the Arowana and Gar appeared beforehand. When I got the Petaltail Dragonfly, I also caught a large number of other rare bugs and fish as well as I went by - and I had good health luck when I did. The King Salmon came along with a Pike as I walked by.
Also, another thing: Sometimes, even the most common fish can really refuse to show up in a day. When a villager asked me to catch, say, a Pale Chub, I fished over 2 hours in my town without a single one showing up. I went to an Island tour instead to get one. This actually happened to me in other common fish requests, too!
This tells me that the rates of rarity change from day to day. Don't get too discouraged!