Are you certain the thread is secure in the tension discs. I haven't had this problem but I do know metallic threads need a special needle with a large eye, mainly to stop breakage. Also there can be an issue with thread twisting.
The other alternative is to put metallic thread on a bobbin and same color normal thread on top, and sew from the back. If you can't see the outline from the back I would suggest stitching the outline in a matching color so you can see where to sew from the back. You may need to decrease tension on the bobbin.