Swipe down on the notification to expand it and you'll be able to see the previous button. It's only visible in the expanded notification, not the summary version. As far as I know, there is no way to change this.
Note, however, that this does not appear to work if you have the lockscreen set to hide "sensitive" notification information:
[...] I have the lock screen notifications setting set to hide sensitive information, and thus it's locking out the full music player controls. When I swipe down, I get the unlock keypad, because Lollipop wants me to unlock my phone in order to get at the full music player. [...] – Derek
Left is the "before" picture, right is after swiping down on the notification:
Click either image for full resolution