Try the test that the TFSR.org docs show. I demonstrate it at about 12mins 55 seconds in this video on YouTube:
https://www.youtube.com/watch?v=MwiI_BXanJ8 (I have the volume off at the moment, DH is sleeping so I didn't check if that's mid-word. I just know that my one ounce weight shows up right around then.
)
ETA: I should also mention that this style of bobbin case is the only style I've had to replace the tension springs on because they deform and stop holding tension. Most other bobbin cases I replace them because they are broken or lost.