One thing that comes to mind is that you might have your attention divided when solving these problems. Let's say there are twelve steps to solving the problem. Within each step, there are multiple sub-steps. What may be happening is that you're concerned with not skipping a step, and so you're thinking ahead instead of concentrating on the current task, which is adding that negative sign.

This is a classic GTD issue. Take your worries offline, and create a checklist for each type of problem. That way, you can focus on adding the negative sign instead of thinking about the entirety of the problem. Come test time, you're unfortunately going to have to memorize your checklist, but that can't be helped.

Good luck!