Garageband changed output levels

I have been recording live music on my macbookpro using garageband. My band recorded 6 songs, we finished mix-down on one song and after adding effects, etc., adjusted output levels. When we thought we had all levels set I burned a cd from the mix to listen. After listening and determining that lead vocals needed to come up, I went back to the mix to adjust and found all levels were different than what we saved. Nobody had access to change anything, yet all output levels (drums, guitars, vocals, etc.,) were different. Do I need to go back to "square one" to re-adjust and what could have happened to our previously saved track? Anyone else have this problem with garageband?