Most adults who try mewing and feel like it does not work fail in predictable ways.
Inconsistency: Mewing produces results from sustained habit, not occasional practice. Sixteen waking hours of proper tongue posture, every day, is the dose.
Ignoring sleep: Eight hours of poor tongue posture overnight cancels out most of the daytime work. Closed-mouth nasal breathing during sleep is what carries the daytime signal forward.
Untreated nasal obstruction: Allergies, septal issues, or chronic congestion make nasal breathing - and therefore mewing - a constant fight. Treat the upstream cause.
Expecting overnight change: Real change happens on the timescale of muscle adaptation, fascial remodeling, and (in adults) very slow soft-tissue improvement. Setting a six-month minimum before judging results prevents premature abandonment.
Treating mewing as the only variable: Mewing works best paired with overall facial health - nasal breathing, sleep quality, posture, hydration. It is a force multiplier on the broader picture, not a single-variable miracle.