Well, I saw this thread and had to check it out. I don't quilt but I do sew every day as part of my 'retirement' business. In reading the 15 pages of posts, I didn't really where see the the original question was answered. I did see that there are some beautiful quilts being made and some making real progress in the posters free-handing skills.Nice work.
If you only have one machine I assume the last posts of your practice work means the issue has cleared up but in case it hasn't,
My suggestions were going to be, in order of likelihood:
1. needle bent, even a little- they are too cheap to not change them,
2. wrong needle- the loop may not be forming or have the correct dwell for the loop to form so the shuttle can pick it up,
3. wrong thread (like most of the other posters)
4. re-thread bobbin- some machines get finicky when the thread spools into the bobbin tensioner the wrong direction,
5. Shuttle timing may be off- when the needle hits the shuttle /hook/case, even a small strike, it can cause timing issues.
6. Feed dogs may not be fully disengaged.
I've been sewing since I was 10, made a lot of my own clothes in JH and into HS, made lots of clothes for my 3 daughters including one's wedding and her maid of honor's dress. I have and sew on a vintage Juki DDL-555, Singer, and Bernina, we have two sergers and we are picking up an additional Juki with walking foot this week. Looking forward to that one- I learned back when the price of material was less expensive than the clothes made from them, before the last of the USA megamills closed on the east coast and we started importing our textiles. It is heartening to see that the mills are actually making a come back.