Note: I don't have a Janome... But on my longarm, I would back off the top tension, but you say that is not working?
Have you completely rethreaded the machine? Sometime the thread may get caught on something, maybe not completely seated in the tension disc or hanging up on a thread guide. Do you have one of those thread guide with multiple holes? Like on my Juki, I have a three hole thread guide. I loop it through the first and last, skipping the middle. Maybe just go through one to loosen the tension.
If the thread path is OK. Then maybe you need to adjust the bobbin tension on the bobbin case. The set screw that holds the tension bar that the thread goes under. tighten it clockwise very slightly, like in hourly increments.
I hope this help.
Another thing would be using a loftier batting to hide the tension.