Can't change read-write permissions on DMG despite ownership

Hey there! So, I recently followed these instructions to create a 20 megabyte, 128-bit encrypted DMG on my hard drive in order to password-protect some confidential documents. I have had no problems opening the DMG or editing its contents, but today I discovered that I am unable to add, delete, or edit its contents, as I receive the following message: "The document "___" could not be saved as "___". The volume “Encrypted” is read only."

I'm able to mount the DMG and open the document, but I can't make any changes. First, I tried changing the permissions on the document itslef; I went to "Get Info," but I discovered that the Sharing & Permissions indicates I (the admin) should be able "Read & Write." I wanted to to tinker with this, so I clicked the lock icon at the bottom of Info and entered my password, but--mysteriously--I can't change the permissions settings, even when unlocked. I click, but nothing happens.

Same thing when I try to change permissions for the whole DMG. It says that I have "Read & Write" permissions, but I can't try messing with these settings even after unlocking.

Now, it's possible that at some point in the last couple of weeks I may have changed the setting of the document to "locked"--I honestly can't remember--but shouldn't I still be able to "unlock" it given my admin status? And even if I compromised that one file, why can't I save any new documents to the DMG?

Any ideas as to how I can return my DMG to true read-write status?

Thanks ever so much!

embr