I think I know what is going on here - but before I say anything on that, please confirm that you are in fact backing up to a device (single file) and using NOINIT, and at no time during the process are you re-initializing the file (WITH INIT).

My guess is that you have re-initialized the log backup file and wiped out your previous transaction log backups. Now, you are trying to restore from a previous backup file using the 'current' transaction log backup device which no longer contains the right log backups.

This is exactly why I do not recommend ever using backup devices. In almost every case, the backup device is initialized and the previous backups thrown away. A good example:

- Backup database (6pm) to backup device (initialize file)

- Backup tlogs every hour after that (noinit)

Now, as soon as the full backup occurs again - the previous full backup is gone, as well as all of the transaction log backups taken right up to the point where you initialized the device (file).