Tension too loose, back off a bit. That happened to me when I got my long arm. Just takes lots of practice even though your scared to touch anything, let alone a tension screw. Are you using a 3 piece sandwich to practice on? I did many many of them before I was at ease with it. Have had my machine since 2009, no such problems now. I also found out using a "Bobbin Genie" made all the difference in the world in my tension, Chotilda.com
( spelling wrong but you know them I'm sure) has them and you want the big sized package same as your bobbins you buy. It goes in before your bobbin so your bobbin sits on the Bobbin Genie. I've only used 2 so far from the pkg. I bought. You see the results right off. One lady told me about them and has her first one still in her bobbin case after 2-3 yrs. Give it a try.
Shirley