Some problems are harder to solve than others. It usually stems from a long testing period. Try X, wait. Try Y, wait. If the wait period is decades, you have a

Some problems are harder to solve than others. It usually stems from a long testing period. Try X, wait. Try Y, wait. If the wait period is decades, you have a problem. Broadly, statistics and large sample sets are used to combat that. Human understanding, or lack thereof, for a given problem also contributes to suboptimal theories. Ultimately, hard problems are hard.