To get to the magnets you will find a plate, on what would be considered the top of the cash can, held on by a screw.

Remove the plate and inside you will find another plate that is held in by two screws. Remove the second plate and you’ll find it has magnets all on one side of it.

If they are correctly configured for their denomination then there should be no issue. Of course, if they are not correct them and see if that fixes it.