Mice are too smart for this event

In the beginning everything was good: With two such traps I have caught on the balcony and in the garage at least 20, 25 mice - until a - could again after they had eaten sick free - whatever. And she seems to have told their colleagues how it works: Since all cut back. I do not know how they do it; the mechanism works as it should work well, but you create it, the beasts.
So: Hands off - unless you want to admire clever mice ...