Why do users love your onboarding and still cancel three weeks later?

Hitting that initial "aha moment" used to feel like mission accomplished for us. Users would breeze through the setup say the product was super cool, and then completely ghost us by the end of the month.

We realized that showing someone value once is very different from making them actually open the app tomorrow.

Automated welcome emails didn't really fix it either because people just ignore inbox clutter. If your users love the first experience but slowly drift away, what specific habit forming trick or workflow loop actually brought them back daily?