There have been several issues reported where this behavior takes place. There are a couple things that might be a problem off the top of my head:
1. Is the system running Windows 8 or 8.1? If so, are you using a microsoft live account to log in to Windows? This will cause the software to fail when registering a user account, so when pre-boot authentication should be happening, it sees that the drive is encrypted but there are no valid users, so it can log straight through to the Windows login. This occurs in part because the system is expecting a domain user in a managed environment.
2. What type of hard drive do you have? Hybrid drives often present this problem, and it may be due to the system trying to read the user access list from the SSD part of the drive, and failing to locate it, then not properly checking the rest of the drive. It also may be caused by the system only accessing the SSD part of the drive when trying to automatically create the user account, which then fails to add the user appropriately. So far we have been unable to get much information regarding this issue, as some hybrid drives work appropriately, and others do not. In some cases, adding the user account through the command line can fix this issue.
In either case, I would suggest decrypting using the client administrator account in the command line, and getting Symantec Encryption Desktop 10.3.2 mp6 instead. The command line guide can be found here:
http://www.symantec.com/docs/DOC7716
The command you want should look something like this:
eedAdminCli --decrypt --disk <number> --au <AdminUserName> --ap <AdminPassword>
For example:
eedAdminCli --decrypt --disk 0 --au ClientAdmin --ap Password