You need to vary your fishing locations. Like in Sims 3, you'll have some ocean fish and some freshwater fish. Depending on the packs you own, fish might only be in certain worlds; there are a couple of vampire related fish that are only in Forgotten Hollow. Some fishing areas require a higher fishing skill -- you'll get the "fish here are too wily" for your Sim notification.
Carl's Sims 4 site has a great guide to fishing -- can't seem to load the site right now but I'll try and grab the link later.