Clark's side, for whom Anthony Pilkington fired a brace, scored a stoppage-time equaliser through Nathan Clarke to make the final score 3-3 after 90 minutes and force a penalty shootout, but Chesterfield kept their heads to win 4-2.
Clark, unhappy with the goals his team conceded after watching his side dominate the first half, said: "We need the players to stop making individual errors that are costing us goals.
"For the third goal, we gave away possession on the edge of our box, it's football suicide.
"We are taking one pass too many when we should be shooting. It's another loss and that's the bottom line - we've lost the game and it's frustrating."