After enabling File Vault, OS X doesn't mount disk automatically anymore
I have had the same issue.  If you have an entry for the drive in your Keychain, delete it and try again.  

To do so, open Applications » Utilities » Keychain Access.app and look for the Encrypted Volume password. Delete it from the list.



Eject and plug in the drive again. When it asks for the master password check the 'remember' button.