Scheduled playlist don't start in the right sequence

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I have the following probleme:
I have a radio station for children. Everey hour i start with a tale. This is a scheduled playlist with a lot of tales,and in every hour the playlist plays only one randomly selected file. That is okay. But before this playlist i have another scheduled playlist, just before one minute! This is a signal for the tale.
For example:
10.00.am - Signal for tale
10.01 am - Tale

But sometimes it happens that centova cast plays the tale first,and than the signal. Why could this come for?

Thanks!