1) Spending more time reading than fishing.
2) Buying cheap gear, thinking they’re just starting out and dont need to buy top of the line gear. I reckon I could bet R. Nadal if he were using a $40 racket. (I could prolly beat him anyway)
3) Fishing solo while learning.
4) Fishing the wrong depth.
5 Fishing unproductive water.
I am far from an expert, but IMHO success comes from practice and patience or someone in the know with the time to help you out.