My Janome dealer told me to never use canned air on a machine that has a computer in it. You'll only blow
dust and moisture into the computer area. Not good for computers. I use a small brush to clean out the
dust in the bobbin area.

When I have a problem like this, I usually change my needle and the problem is solved. If changing the needle doesn't solve it I clean out the bobbin case.

I have that happen when I need a new needle.
Maire

The only thing I use to clean my machine is a long pipe cleaner. It picks up the tiniest amount of lint. I've been complimented by my mechanic several time for how clean my machines are. Last time he was here he said my machines were the cleanest he's ever seen. Hence I only need to have them serviced every two years. I do clean them regularly but not at every bobbin change. He also said to give canned air a big miss.
This guy was head mechanic for Janome for 15 years here in Brisbane.

The teflon washers in the bobbin case sometimes helps. When I worked in a sewing factory, we always wound the thread twice around the guide when we were winding the bobbins to use. After the bobbins are wound, test the tread with your fingernail. If your nail sinks in easily or the wound thread seems a little spongy, it is wound too loosely. (spelling) The tread needs to feel a little firm. This can cause backlashing.

While I agree with all the advice about cleaning I do have a question about thread. Since you used two different colors of thread it can't be that particular spool. So, the next thought, how old is the thread? Are both spools the same brand? Are you using the same thread on top and bottom? Are you using serger thread instead of sewing thread? Are you using a coated thread like hand quilting thread by mistake?

Thread can play an important part of your quality of stitches. If all else comes back been there, done that, then I'd be looking at the fabric you said is 'stiff' and/or batting that has lots of imperfections.

Hope you figure it out before your next project!

I would check to make sure that I have the same thread in the top & bottom & I agree with the others not to use canned air on my machine. keepsake or one of the quilting catalogs has an attachment to vacuum out your machine. i was told by my Bernina dealer when i first bought my machine not to even blow in it because of the moisture. good luck i hope you sew problem free tomorrow

I wonder, could it be batting that is "bearding" thru and catching the thread? I can see some white dots of batting showing thru on both photos. While I would normally agree with cleaning the machine, particularly the bobbin, the fact that it happened on two separate machines says it's got to have something to do with the sandwich.

My question is did this happen on each machine after you had been sewing for awhile? If so you may have a small piece of lent somewhere that is messing things up.The fabric you stated was not top quality , some cheaper stuff does have alot of fibers in them.Also when fabric is made sometimes at factory we sprayed cleaners on spots that your neddle may be getting a slight coating and causing a ever so slight pull or gumming up your tension.

That fabric looks like it would take at least a size 14 needle. Not using the right size needle for the fabric you are sewing on will definitely cause that kind thread nest on the back. Just my opinion and from my experience also.