Sounds like maybe your bobbin thread tension is too tight. The bobbin thread travels from one side of the zig to the other zag, if it is too tight it can cause tunneling. I put the same thread in the bobbin as in the top spool and ease off the bobbin thread tension (bobbin screw -tighty righty or lefty loosey) until I get the look I want on a scrap. If a little of the bobbin thread ends up coming up, ir doesn't show because they are the same thread. Good Luck!