There you stand knee deep in water. Thirty feet away, you see the splashy rise of a trout. You swiftly pull out line and punch your cast where you think that fish was. The fly rips through the air and splats down. She floats by and…nothing. You repeat the process several more times - nothing. You shrug it off and assume that fish just didn’t want your bug.
A more likely assumption is that your fly was nowhere near that trout.