Did you book directly with DL or via another party? If someone else, it's likely the info wasnt sent
If with DL there's an issue periodically where ones Precheck number disappears. I'm not sure we ever came to a conclusion why, but it does randomly happen. Go into your Skymiles profile, if it shows up remove it, save, and then add it back and save. If it's not there, simply add it back and save.