Two failsafe methods. Firstly put Nutella or some other chocolate spread on the trap, as they have to make an effort to remove it, it triggers the trap.. Secondly, if you have more than one mouse, and in my experience they are rarely on their own, put some poison into the toe end of a nylon stocking and mix with bird seed, tie into a bag and put down where you know they are. As they devour the seed they also ingest the poison. We lived in the countryside and as soon as the weather cooled we would get these visitors. However, we didn’t make them welcome and got rid of them within a couple of days!