Each stat has current and maximum numbers. When they match, you cannot upgrade this stat further until you increase the max limit.

Custom reels, lures and lines increase the maximum stats. Collect these parts from Fishing Crates to unlock attachments and then equip from the Customize screen.

Remember - when customizing your rods, you can only attach one reel, lure and line at a time, so always use the best ones you have.

Note: Usual and Elite rods share attachments, so, for example, Ozarks Classic Reel IV can be attached both to Poseidon and Classic rod!