I agree. This happens to me when I stop and reload the bobbin then I forget to put the pressure foot down and wonder what happened...

I also had trouble when I haven't changed the needle often enough. But, all the suggestions I've read are good things to check. This is definately the place to ask!

Another reason it can happen is when your quilt is loaded on the frame but not the right distance from the machine plate, also make sure all sides are same height. Not sure if your frame has four adjustable rails or not. If your tension on the thread is correct and when you do a practice piece NOT on the frame and there are NO loops but on the frame it happens it is the tension of the fabric. I hope this makes sense.

Thanks will check all these constantly!

It is not possible to use the Janome 1600P without lowering the foot. It is a fail safe. I have this machine and the only time this happens is if the bobbin tension is too loose on mine. I have another semi industrial and it did that when the top thread came out of the tension wheel. I cured this by using all three holes on the thread guide. This may work on the Janome too. Worth a try.

Great idea to use mirrors to see the back while quilting. Better than getting down on my knees and crawling under. LOL!

Presser foot not down? Out of the tension discs? Been there, done that over and over

mine is always because something has come unthreaded somewhere.

this is just a thought, FWIW LOL, is your bobbin spinning...I know I have this problem on the quilter...I use the little plastic rings in the bottom of the bobbin case...

I hate when this happens!! Check your threading and make sure the thread is flossed into the tension disks and threaded into all the guides. Also make sure your bobbin is in the bobbin case correctly.