This type of problem is sometimes caused by thread debris getting caught between the tension discs. That holds them open, so you can’t get proper tension. You can probably fix it by following these steps:
1. Lock the machine. That opens the tension discs.
2. Take a narrow strip of fabric and fold it in half. Put the folded edge in the tension slot at the top of the machine and work it back and forth like dental floss.
3. Thread the machine all the way, but stop before threading the needle.
4. Unlock and sew a couple of stitches, then stop, leaving the presser foot DOWN.
5. Pull on the free end of the thread. You should feel resistance. If you do, thread the needle and go. Otherwise repeat from step 1.
If that doesn’t solve the problem you’ll need dealer assistance, but it should not require going back to Janome.