I've been using these for quite a while now and I've never seen them smoke or steam like you mentioned earlier. They are warm and when I replace them the wicks have changed to a darker color but I don't think that it's anything unusual.

I sure hope that you'll be able to find out what's causing this so that you can start using them again. I use them to help ease stress in my household and I can tell when they're getting low because my cats will start acting up and fighting with each other more. Good luck.