Hi Stan,
The secret is sometimes in the "contents" of the key file. Keys come in various flavors, and can contain multiple parts.
Use notepad to open the one that works, and see what's inside.
Then use notepad to see if you can find the same sections in the other key.
Just changing the file extension isn't the right thing to do, often you have to "extract" one bit from inside the supplied key. It's all text though so you should be able to follow what I mean.
Cheers
Bruce