It is Definitely subwoofer placement, but I think simplifying it by advising "just put it in a corner"; Is the wrong impression to give someone less knowledgeable. Impediments such as windows, doors, frames and moldings may offer a decrease in performance. Thus proper testing and upgrades to the location in question (panels, calibration and auto calibration systems) also have a large stake in a desirable output.
The two most important factors (a little broad)
1. Placement
2. Calibration