They are nasty little creatures. Much smarter than they are given credit for. It's because they stick together. One goes and they all go. I wrestled with them for half an hour two days ago, and just when I thought I had tamed them, they proved me wrong.
What is wrong? I tested everything. Jiggled every tiny bulb. I finally gave up and started over with a new strand. I checked it carefully before hanging them, so as to avoid any more problems, but as soon as I finish, wouldn't you know it - 3 burned out bulbs. I nearly just left them that way, but decided to press my luck and change them.
They are currently working. They are hanging in my family room window, and if I tilt the blinds a half-turn backward they all blink off. I wish I had never done the lights. My sanity is a very delicate balance these days, and the lights just may be the end of it.