White's move 12 indicates that he is looking to open up the a-file with pawn play, rather than fix Black's weaknesses and exploit them with his knight. With some help from Black, White executes an advantageous piece trade (knight for bishop) on e6, but then errs by also trading his dark-square bishop for Black's. After seizing the a-file with the help of his monster Bg2, White is unable to exploit it after Black redeploys his forces to prevent further penetrations.
I've previously run across this problem in the English, where I'm able to achieve a large queenside space advantage, but then lack the wherewithal to make any further progress. The improvements and other potential avenues of play found in analysis (particularly on moves 12 and 19) should help me overcome this problem in the future.