It took me a couple of years to diagnose this problem, which was happening well before the 15000, on the 9000 and subsequently the 11000. There are two stages to the problem. It starts with the bird nest on the bottom, which many can identify with. That happens because the stitches going around the bobbin are not being pulled up tight. New owners often have the problem because the thread does not actually enter the eye of the take-up lever, whose job it is to pull up the stitch. There's a little clip on that piece that is supposed to prevent the thread coming out. If you thread quickly you may not notice that the thread didn't get past the clip, and instead just slid down to the back of the lever. It looks fine, but will jam very early in sewing. Sometimes the thread will loop over the top of the lever, and snap into the clip. This causes the thread to become OUT of the lever, and again trouble happens. This is all more or less normal, and we've all had the problem at one time or another.
What happens during the creation of the bird nest is the critical bit. Usually it's nothing, you can stop sewing, fix it, and move on. However if enough loops pile up under the bobbin case, it will be lifted up to the point of spinning past the stopper that's supposed to prevent that. This usually brings the needle down into the case, punching a hole or breaking off. That's bad enough, but sometimes the needle will deflect enough to hit the hook race, with is the round metal "basket" that the bobbin case rides on. Because a magnet is used to hold the bobbin case in position, that part is aluminum. If the needle hits it, a burr will be raised. Sometimes a chunk will be taken out of the edge. In either case, even if the bobbin case is replaced, stitches will tend to catch on the burr. If they don't get freed early enough in the cycle, they won't pull up and another nest will form. That, in turn, can cause additional damage.
Once I realized what was happening, I asked Janome to make something that could buff out the burrs on the hook race. They created a tool, part # OILSTONE. It's basically a round ceramic stone sized to fit the hook race. A technician can use it to smooth out minor nicks and scratches, but in severe cases the hook race itself should be changed. I retired from selling machines in 2011, but up until then this problem was not addressed in dealer training. Most dealers never look at the hook race in doing a service. Once I was aware of the problem, I started inspecting the hook race on every machine that came in. That saved me a lot of headaches by preventing trouble before it happened.
Sending the machine back to Janome is not necessary, and I'm not altogether confident they would correct the problem. With a small staff, lots of machines to service, they don't have time to do much more than replace broken parts, do a short test sew, and send it back. Instead ask your dealer to do this:
1. Order the part OILSTONE. It will be a big help to him or her.
2. Order a new hook race and bobbin case stopper. The bobbin case itself should be in stock, but it too should be replaced.
3. Replace the hook race. It's a bit fiddly the first time, but can be done without re-timing the machine.
4. If the bobbin case stopper shows signs of having a bent spring, replace it. Be VERY careful to adjust it so that it goes far enough toward the back of the machine to stop the case, while just allowing the thread to clear.
That should get your machine going again. If it's frozen, as in not turning, it is likely due to the hook being embedded in the bobbin case. Turning the wheel backward a bit usually pulls it free. Then you can remove the needle plate and bobbin case. With those out the wheel should turn freely. If the machine doesn't turn on, it likely blew a fuse. That's rare enough that a replacement probably is not in stock. But dealer parts orders usually can take no more than a week or two, which is MUCH faster than a shipped machine turn-around. Once you are going again, take care to make certain that the thread enters the take-up lever properly. When you are embroidering, be especially mindful of the design and the needle. If a design has a lot of tiny stitches, commonly used for outlines, a size 14 needle can cause serious trouble. Size 14 means 1.4mm in diameter. If a stitch has a length of 1mm you will have multiple stitches going through the same hole and that will create a bird nest. For very short stitches try to use the smallest needle possible, and consider 60wt thread instead of the usual 40wt.
You can print this response and take it to your dealer with the machine. Some dealers will already know, or will welcome it. Others may take it as an insult to their skills from some old buzzard who isn't even in the business any more. A plate of brownies brought in with the machine may improve your odds in such cases. Best of luck, and let us know how it turns out!