I already have a LightScribe disc in the disc drive, but LabelPrint keeps telling me "There is no disc detected in the drive". Why?

Please go the LightScribe official website to download the latest LightScribe drivers. After updating the LightScirbe drivers, please reboot your computer and then try again.
http://www.lightscribe.com/downloadSection/index.aspx
Note: Make sure the disc is properly inserted in the drive. The label side, which contains a special coating, should be face down in the drive.