A common problem for me.
I think it has something to do with how you move around on the board, or whether you "back up" to the table of contents rather than hit the button. I haven't figured out a pattern.
A quick workaround that works for me: click on another board (that is, if you are on CE, click on the "Off Topic" switch). When you are over there, click the switch for the board you were originally on. The "Earlier" button will usually reappear.