My bet is that there's a problem with how you have inserted the bobbin. Could be that you have the thread coming off the bobbin clockwise instead of counterclockwise (or vice versa), or that you have missed a threading notch in the bobbin area. That is what I would focus on first. Pay very close attention to the diagrams in your manual for how to load the bobbin.

I have two different machines and not using one for a while I have to get out the manual and follow it exactly. Usually it is because I out the bobbin in backwards, thread going in the wrong direction. make sure when you pull the bobbin thread it turns in the right direction, like the picture shows.

Clean it all out, CHANGE THE NEEDLE, rethread. I have a Janome embroidery machine and has had nothing but this kind of trouble. Sometimes I can fix it on my own and sometimes not. It has been in the shop 10-12 times for the same problem as you. Janome has a problem with the bobbin. It is noted in several blogs. Janome claims they have corrected the problem, but they haven't.

make sure you have the bobbin in with the thread coming off correctly!

That's interesting ... I've had four Janome's, and have never had problems in this regard (or in anyway, as a matter of fact).

Now I'll just hope that I don't jinx things with this! :)

Is there a chance your feed-dogs were inadvertently dropped without your knowledge? If you continue to have this problem after checking threading paths and bobbin insertion, you may want to take it back to the store for an exchange because it may not be something you are doing. Dealer should either resolve quickly or replace the machine with a new one. Also at any point did you remove the bobbin CASE? if so, that can be finicky to get back in just exactly right and it is difficult to tell if it isn't in right because it looks as if it is. Last suggestion is could your needle be blunt or bent and is it inserted all the way up? Let us know what problem is when you find out.