There are a couple of reasons why your croutons lose their crunch. First, ensure that your bread cubes are evenly spaced in a single layer when baking. If they're touching, they won't properly crisp and will end up soggy. Next, make sure you're storing your croutons correctly. Before adding them to an airtight container, make sure that they have completely cooled. If you add your croutons hot, it can trap moisture in the container and make your croutons soggy.