There are about three things that cause birdnesting: Unbalanced tension, flagging, or a problem with the hook assembly.
For tension problems, check the thread path to insure the machine is threaded properly and then sew a tension test such as a 18mm capital "I" test.
For flagging , be sure the fabric is framed tightly. Also, make sure there is not a significant gap between the hoop assembly and the sewing arm.
Third, if there is a problem with the hook assembly. Either the hook positioning finger is mounted too close to the hook basket, or the hook basket does not allow the sewing hook to spin freely
Reasons and Solutions:
Bird nesting typically occurs when the bobbin is not set properly: whether the tension is incorrect, the bobbin case is dirty, or the bobbin is not properly placed in the case.
1) Remove the excess thread from underneath the needle plate. To do this, you will probably have to cut away all of the nested thread with a pair of scissors.
2) Remove the bobbin and case from the rotary hook.
3) Remove the bobbin from the case.
4) Ensure the bobbin case is clean. Please refer to Bobbin Case Maintenance.
5) Once the bobbin case is clean, reinsert the bobbin and ensure the tension is correct. Please refer to the Bobbin Adjustment section under Tension Adjustment.
6) Next, you want to make sure the pre-tension is adjusted correctly. Please refer to the Pre-Tension section.
7) The last tension adjustment you want to make is to the main tension. Please refer to the Main Tension section.