have you asked the dealer or distributor or help line for your janome? They've heard all the possibilities and can probably trouble shoot about that particular machine better than anyone else.
Get out the owners manual.
Common problems include:
Tension: make sure the thread is properly seated in the tension disks.
Cleaning: get a magnifying class and flashlight-be a detective
Oiling:
Is this a new or recurring problem?
Then call the help line.