Symantec Drive Encryption no longer supports Bootcamp after Apple made changes to it in Mavericks. Fusion drives are also not supported.
Your drive may also have been converted to Core Storage during an OS upgrade, which is also not a supported configuration for Encryption Desktop.
Lastly, if another encryption algorithm is detected as in use on the drive, the drive will not appear. Typically, this occurs with OS-based encryption that is turned on either by an OS update (in my experience I have seen this on OS X a few times), or preset as enabled by default (more often on Windows).