Screentime app limits don't work

Screentime app limits keep disappearing from my childrens iphones. I have set up everything correctly, all phones (mine and childrens) are up to date, I have set up a passcode to change limits, but every other day whether the app limits are gone, they just don't work (I see a limit is on, but a child can still play) or the limits are on but can be "overplayed". What can I do to make the limits work? Its very-very frustrating.