What is one of the potential causes for beer not flowing from the lines?

The closure of the CO2 main valve is a critical factor in draft beer dispensing, as it directly impacts the pressure necessary to push the beer through the lines from the keg to the tap. When the CO2 main valve is closed, the draft system is unable to maintain sufficient pressure, leading to a disruption in flow. Since beer relies on carbon dioxide to create the pressure that propels it through the lines, a closed valve results in either no flow or very limited flow.

In contrast, while the other options may affect beer quality or specific aspects of the dispensing system, they do not directly cause the beer to stop flowing. For instance, if the coupler is engaged, this means the beer is properly connected to the keg, which allows for flow once other conditions (like pressure) are met. Temperature can impact the pour quality and beer management in general; however, warm beer does not prevent it from flowing. Similarly, insulation of a jockey box may affect the cooling efficiency but does not interrupt the flow itself. Thus, the CO2 main valve being closed is a definitive cause of no beer flow.