In single-subject research, what is the typical conclusion if a treatment effect remains difficult to detect visually despite strictly controlling extraneous variables?

A researcher conducting a single-subject experiment notices that the treatment effect is weak and inconsistent across phases. Rather than switching to a statistical test to confirm the small effect, the researcher decides to tighten control over extraneous variables and modify the intervention to produce a stronger, more visually obvious change. This decision is consistent with how single-subject researchers typically handle data that is difficult to interpret through visual inspection.