You are encountering a multisession problem. type in "multisession" in the search box and you will find many many post in here addressing how to get that last track off a multisession disc. The quick method is to go to the import tracks tab. use the self naming function and begin the import. When it gets to the last track it will start the import and approximately 3/4 of the way through you will see the progress stop and you drive will start to make a sound like it's confused. At this point, eject the disc! Now hit the move imported track to custom assembly tab. now open the custom assembly tab. you will see your first 14 tracks there. Now, on the select track from hard drive, open it up and you will see all the tracks that you imported, includung the 15th track. Select that track and click OK. Now all fifteen tracks are present and ready to burn. Then you write all 15 to a disc and you are done. Like I said, seach "multisession" and you will see other detailed post about how to do this. I hope this helps.
