Hm. Not sure what you are talking about.... unless you are trying to play on another game mode like Normal, Hard or Crushing.
If you play and finish the game on one mode, you can only play Chapters, use Bonuses, etc, on THAT mode. Meaning if you played it on Easy, then you have to play Easy again to play Chapters. You have to beat the game on each mode to be able to play Chapters on those modes, respectively.