Hey there, Caught your message.
I feel your pain, and trying to get support out of SWF is like smashing your head against a wall. If you bought a used one, then you might not have SWF support available to you.
What you are experiencing is referred to as a thread "pull out" I used to own a bunch of SWF's. I have found over my years that this is a VERY common problem with them. Typically speaking SWF i've found likes to run tight, the optimal tensions are usually tighter than most machines. But if your H tests look good, then it shouldn't be a huge issue.
Which 15 needle do you have? Is it the one with the color LCD display or the one with the LED?
Some things to check:
Check the picker. Its that little fork looking thing in front of the bobbin? Is it bent? It should be straight and even. If it is not. Adjust it so it is with the screws on the side and make sure it just barely clears the guide screws on the side.
Increase your tail length, if you have the LCD it is under picker off timing, the LED it should be under Paramters (I believe, don't really remember that one)
Make sure its CLEAN, take off the needle plate and make sure there is no excess bobbin fibers, thread pieces etc, if so, you could be interfereing with the cutter.
There could be numerous other things.
Go to
www.swfdtg.com and click on general FAQ's. I believe they have a PDF you can download / print with all the steps that could be causing this.
Hope this helps.
Any other questions just let me know.
-- Eric :-)