Welcome to the wonderful world of embroidery....sometimes it is like the nursery rhyme: once there was a little girl with a little curl right in the middle of her forehead. When she was good, she was very very good and when she was bad, she was horrid! Some days are just like that.
First, check the manual for your embroidery machine to make sure you are using the correct weight of machine embroidery bobbin thread.
Second, make sure the bobbin thread is feeding in the correct direction from the bobbin spool.
Third, make sure to have the presser foot up when thread any sewing or embroidering machine. Having the foot up opens the upper tension feature of the machine so the thread can properly enter this feature of the machine.
Fourth, any adjustment to the upper tension should be done with the presser foot down.
If all the above has been your SOP (standard operating proceedure), then the bobbin case tension may need to be adjusted. Before doing this, test the design again with a different fabric, new needle and correct stabilizer.
To adjust the bobbin tension, mark the top of one end of the screw slot with a tiny dab of nail polish so you will know where the original setting is. Then move the screw just a tiny bit to the right for tight or left for loose. Tighter will pull the top thread slightly to the bottom of the design.
Please let us know how this works out for you.