Your situation
You said: they know it, and they still do not do it.
You said: they do it once, and then they slip back.
You said: they do not do it, and they tell you why.
You said: you have tried all sorts, and it keeps getting stuck.
And it is about customers, citizens or patients. You do not see them at the moment they choose, so that moment is what you design.
And it is about colleagues or a team. Then what the rest of the team does is your strongest force.
And it is about a whole organisation. Start with one behaviour in one place. An organisation changes one routine at a time.
Knowing and doing are two different systems.
People have the information and the good intention. But in the moment, another part of the brain decides. Fast, automatic, on what is easy and normal. Your message reached the conscious part. The behaviour comes from the automatic part.
Tomorrow, find the exact moment when the behaviour has to happen. Look at what gets in the way there: an extra step, a colleague who does not do it either. That is where you start. You learn this in two days.
You designed a first time. Not a second.
The first time came from attention: a campaign, a request, a fresh start. Then people went back to the same meetings, the same defaults and the same colleagues. And that environment beat the new insight.
Tomorrow, look at what should have prompted the second time. And whether anything rewarded the effort straight away. Usually both are missing. Repetition can be designed. You do not have to hope for discipline.
They are telling you exactly what is wrong. Believe them.
When people say it is too much effort, it is too much effort. Small hurdles weigh more than big benefits: a form, an extra login, an unclear first step. Pushing motivation up is expensive.
Tomorrow, walk through the behaviour yourself from start to finish. Count every step where you hesitate. That is where people drop out. Remove one, and see what happens.
You intervened without a diagnosis.
Information, rewards, sanctions: every attempt assumed something about why people do not do it. If that assumption is wrong, you pull the wrong lever. Maybe it is not motivation but effort. Maybe not knowledge but what everyone else does.
Tomorrow, plan no new action. First map what holds the current behaviour in place. That diagnosis tells you what will work. Only then do you design.