I asked for help here, recently on my "nesting" problem on my 4 month old 15000.I had lots of very helpful and informative answers, but I haven't managed to cure it and have now discovered that absolutely no bobbin thread is showing on the back of the embroidery. It is all top thread. I got through two whole reels of embroidery thread yesterday, trying to do a 12000 stitch embroidery. Do I have to give in and send it back for repair???? I'm soooooo sad! I have tried the usual - passing fine crochet cotton through the thread mechanism, folded calico down the thread path - nothing helps. Can anyone help before I take the plunge and return it to my dealer who is 500 miles away. He will collect but I will be without my machine for a couple of weeks. Also, I'd like to know what I did wrong, if anything. Whatever it is, I can't afford to do it again.
Being way more than 500 miles away, Jim types loudly so you can hear this:
The issue is tension. Either there is too much bobbin tension, or not enough needle tension. Bobbin tension is fixed, although you do have two bobbin cases. The red dot case is the standard, while the yellow dot case has extra tension for use with fine bobbin thread, if needed. If your problem is happening with the yellow dot case, change back to the red dot. What happens with regular sewing, such as a zig zag stitch? Do you still have a straight line on the bottom, or does the zig zag look OK? Have you made any changes to the tension in the SET screens? If the top tension has been reduced there it would cause this same problem. Set it back to default, or use the + option to tighten it up a bit. Debris caught in the tension can hold the discs open, reducing or eliminating top tension. That's why you ran the calico through the top tension path, but apparently it did not help. Note that you must do this when the tension is open, which happens when LOCK is engaged. You might try it again in that state.
If all else fails you should have the dealer collect it for service. Being without it for two weeks probably won't be any worse than the weeks of frustration you've already endured!