I also have a Janome QC6260 and it lets me FMQ fine. I did recently have a problem with thread nests underneath and traced it to a burr - a scratch - on the bobbin case and another burr on the needle plate. They weren't big - just enough to catch up the thread when it tried to stitch. Other than that it's been smooth sailing - after the learning curve.
Try watching some videos from Leah Day's site - freemotionquilts.blogspot.com.
I use the darning foot - if it's not close enough to the fabric, you can lower it by adding a wound up elastic to the top - between the top of the spring and the top bar. Make sure the pressor foot is down (I forget this all the time.) Definitely take your bobbin thread up to the top of your fabric before you start.
I have my speed bar set at medium - that way even if I press the pedal to the metal - it only goes so fast and no more.
Make sure you have a good bobbin with no burrs or rough spots.
Use a large enough needle - at least a 90/14. That way the thread doesn't take as much friction going through the fabric.
Check what thread you're using. For practise - use a good cotton all purpose thread or a poly/cotton mix. Don't use the fancier 40 weight threads when you're learning. They are thicker and take a bit extra care to get the tension and the speed right. Some of them are quite soft and prone to breakage.
When you're practising, use plain fabric with no seams. Sometimes the seams if they're thick can break your thread.
Start from the beginning.
Read the manual.
Set the thread tension that's good for straight stitching.
Do some FMQ straight lines, then try slightly curved lines before you try circles and swirlies.
When you do circles, pay attention to when you're pulling the fabric backwards under the needle - if you pull too hard you'll put tension on the needle and it may bend, or break, or be just enough out of alignment that the stitch won't form properly.
And as people have said, practise, practise ,practise. If you're still not improving, try taking a class or two - perhaps you have picked up a habit without knowing that is causing problems.
Hope you find something in here that helps!