Yupp completly agree with the guys that are saying its the thread. I had this very same thing happen to me about 8 months ago and after eliminating things one at a time I found it was the thread getting contaminated because I use to keep it exposed on the bench in a thread rack.

Now I keep all my thread in one of those stand up plastic storage containers that you can buy at any hardware or Walmart so that the thread spools are protected from anything in the air (cork dust, dust, chemicals). Since I've done that I havent had any problems.

Steve