I've found vintage Elnas can be a little picky with thread. They like thin thread and thicker thread can cause tension problems. I've set all mine to use the 50 wt. thread that I most often use, which typically means I have to change the bobbin case tension to lower than the factory set tension. I don't know what kind of thread you're using? But, it's easy to change the bobbin case tension on supermatics with that lever that has the red dot on it in the case area. It has a slot for a screw driver. You can change it up and down to see if either makes you happier with the tension.
Most vintage machines will need to have their top tension lowered when doing zig zag or embroidery stitching. That's just how they operate.