DMG password protection under Mavericks Under Snow Leopard I applied password protection to some archives on a Flash drive. Upgrading to Lion and later Mountain Lion did not affect that protection, but Mavericks strips it from some (not all) of these images. Plus, the Images menu in Disk Utility no longer includes the "Change Password…" item. Any suggestions, please?
Under Mavericks, if you want to change the password, you need to convert the volume. 

Your problem with the passwords is probably that they might stored in your keychain, therefore it won't ask you for it on your computer. 

You can remove the stored password in the Keychain Access.app.