Trouble recovering EFS-encrypted files

I'm on a Windows Vista computer. I've wanted to use EFS for a little while but I wanted to experiment on some individual files first. While doing that, I backed up my certificate and key, reset my password, and imported the old key. Everything was fine and I could access the files that I experimented on.

After I was convinced that it was safe, I decided to encrypt most of my files. I imported the old key and certificate that I had previously backed up before doing so. When I was done I reset my password just like before and imported the old key again, but this time I can't access any files. I've imported it several times but I continue to get "Access Denied" for any encrypted file.

So what happened? Did Windows use some other certificate to encrypt these files for some reason? Is there any other possible reason that this would happen? In other words, is there any hope to recover these, or has the key just been lost?

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