I've been fighting this problem since I got my machine, JanomeP DBX. I recently read that the DBX takes a rounded topped needle and not a flat topped one. And preferably sz 18. I'm anxious to see if this helps, the new needles just arrived. I've been so discouraged I've even offered it for sale, then decided "Just one more try."

I never believed this could be true but my Babylock doesn't like Walmart but loves Joannes. That's OK. Gives me an excuse for a shopping trip.

I was just piecing a quilt the other day and my thread kept breaking. I changed my thread and it stopped being a problem. Maybe it's the thread.

Sometimes if your bobbin is filled TOO full it will break too. I found this out by trial and error.

Change needle, re-thread machine, re-wind spool, and if all else fails try another type/brand of thread.

Check to see if there might "burr" on your bobbin case. Mine did that and the repair man found a tiny "burr".

Well I will be trying a different thread but will be a few days before I can do it. Also will check for burrs on the hook, however it seems that the thread is fraying where it passes through the needle have also had some problems with skipped stitches. Otherwise the machine stitches wonderfully but I have got to figure out the thread issue.

The needle size for a 50 wt thread is 70, not 90.
Anyone know the tex (measure of strength) for star thread? I think (don't quote me) it's not very strong.
Needed to use a lower tex thread in my LA (due to color) and had to go very slow.

Janome and thread
 

I had been having real nightmares trying to FMQ on my Janome 6300. Bobbin washers really help. They seem to work well to prevent the bobbin backlash thing that makes thread break. A really experienced quilting friend suggested them to me when I was pulling out my hair over trying to FMQ a heavier flannel quilt. Bobbin washers and tension tweaking - do change your bobbin tension to a lighter setting and then work with your top and bottom tensions until your machine likes your thread. This doesn't always work out but reading here and on the forums at PR in the quilting section really helped me see that tweaking the tensions for different machines and threads is really something you have to do. I use a #14 Superior Topstitch needle always, and would go up to a 16 or even 18 is using heavier threads on flannel or something like that. I love Superior threads, but so far my machine prefers Maxi-Lock and also likes Superior Magnifico and So Fine. I am working on getting it to like the Masterpiece and King Tut - so we'll see.

I have the 1600P on a handiquilter frame. Mine is very picky about threads. In fact the only thing I have had luck with is Mettlers thread , a topstitch needle, set just shy of 3 on tension with bottom line in the bobbin. Skipped stitches are typically caused by your encoders. Be sure that your track is free and clean of lint and dust. Good luck and just keep playing. I found it easiest to make a journal of what worked and what didn't.