Tom, have you checked the battery and the output? One time I was using mine with an almost dead battery and got terrible results.
Edit,
Another thought is - You said you waited to really use it? Most people experience anything from a little to a lot of die-off when they first start zapping, so just before a cold would only make things worse. Keep Zapping daily 1-3 times a day for 2-3 weeks, then see how you feel. Your body needs a chance to clean up all those dead critters else any new ones you pick up will absolutely thrive on them.