This problem is most often seen on OS X, so the following instructions cover said operating system. The same general principles apply to Windows and Linux if you have only temporarily mounted external drives.
This is normally a problem when you specify a music folder within /Volumes, for instance /Volumes/NAS/Music.
Folders within /Volumes are normally external folders, such as on USB drives or NAS drives. When OS X shows the files and folders within one of these volumes, the volume is said to be mounted.
The underlying problem is that an external volume is not automatically re-mounted when you start your computer. Thus, bliss cannot see the music files. This can have numerous effects. If you have already scanned your music files, bliss will show the library but existing artwork may not be shown. Rescans will appear to complete immediately.
To fix, initially, you need to remount the Volume. This is accomplished by opening Finder and navigating to your music folder. Once you reveal your music files and folders the folder will have become mounted and bliss should be able to see the music files.
The fix can be made permanent by setting OS X to mount the volume when OS X starts. This is accomplished by adding an item to your Login items in System Preferences/Accounts.
An alternative is to not use a sub folder of /Volumes and instead use a UNC path.
Any other FAQs I forgot or clarifications required? Post your ideas below!