They often stick with a loser until it really hurts, then take the loss. Many traders get a fundamental case and hang onto it, even after the market technically turns. Only believe fundamentals as long as the technical signals follow. Both must agree. Let profits run.
Many people trade with their hearts instead of their heads.
That's why they should have a plan first, and stick to it. Often traders have bad timing, and not enough capital to survive the shake out. Too many traders perceive futures markets as an intuitive arena. The inability to distinguish between price fluctuations which reflect a fundamental change and those which represent an interim change often causes losses. Not following a disciplined trading program leads to accepting large losses and small profits.