All roofs need maintenance, especially tile roofs. Over time, debris such as silt, leaves, dirt can accumulate under the tiles at the concealed metal flashing. These areas include valleys, skylights, and chimneys, amongst other things. What you need to do, in order to ensure that you don't have leaks, is remove the tiles around the concealed metal flashing, sweep the debris clean, and then reinstall the tiles in these areas.
Performing this type of maintenance for your roof will not only ensure that you don't have roof leaks, But it will also extend the life of your roof.