I was having problems with that on my 401. It has the same kind of drop in bobbin system as the 66. I thought I had cleaned it well, but I was wrong. The thread was not only making a nest under the fabric, but the thread was looping around as it made a stitch and getting stuck. I would have to cut the thread to get the fabric out. Then I couldn't figure out where it was getting stuck.
This is the thread I posted about it. This is the
thread I watched that showed me how to clean the bobbin area.
bkay