Does anyone buy the compressed air to blow out their bobbin assembly area and needle area? I've been using that and love the results. It blows out all kind of lint.
As to the problem, I would get out my manual for the machine, go back through every single threading procedure, including the bobbin, making sure I had everything correct. I never never mess with the bobbin tension.
If all else fails, take it to a Janome sewing machine dealership and see if they can find the problem. My local dealer helps me with problems like this without charge since I buy from him.