Whenever I fished, It's always "Oh! A bite!" But when I reeled the rod, it always got away...
Why is it so hard to reel in Pokemon? Am I just unlucky? Is there a certain way to always get the Pokemon on your rod?
Starting with Generation III, the fishing mechanic became more complex, as the player would have to pay attention and confirm as soon as the rod hooked a Pokémon, or else it would get away. A series of ellipses appear prior to the text "Oh! A bite!", which is when the player should press the A button exactly once. In Pokémon Ruby, Sapphire, and Emerald, this required a series of confirmations when using better rods, and the Pokémon will also get away if the player presses A or B at the wrong time.
So you're just pressing A button too late. The proper time is when you see those ellipses, which in most basic terms is (...). Additionally, as you use better rods, you have a larger margin of error where you can press A and still feel successfully.
Source: https://m.bulbapedia.bulbagarden.net/wiki/Fishing