Encrypted backup password

I backed up an iPhone to iTunes (encrypted) and it did not ask me to enter a password. I then erased the iPhone, and went to restore it. When I went to restore it, it told me I needed a password. Ok, no big deal, I had my encrypted backup password written down from the last time I needed it, so I entered that. It told me it was incorrect, so I entered it three more times, along with several other passwords since I read online that it may use computer login passwords, iCloud/iTunes passwords, etc. I tried them all and none of them worked. I then backed up a second iPhone (encrypted) and this time it did ask me for a password before I backed it up, so I entered my old one. Then I went to restore it and entered the same password I used for the other phone and it worked and restored the second iPhone just fine. So, I tried restoring the first iPhone again using the same password and it still says the password is wrong. Why?! What can I do to restore the first iPhone if the password won't work? I only backed it up in iTunes, not iCloud or any other program as I've never had a problem before and I thought I knew the password.