Gütermann sew-all weight polyester? It should be the easiest to get the stitching perfect. Further trouble shooting: Tension setting; test sew with straight stitch on two layers of shirt weight woven cotton, then the stretchy stitch pattern. When balanced switch to the jersey fabric and make sure it's newly steam ironed. To get the threads to interlock just so on two layes of light weight fabric layers requires fine tuned tension. Make sure to match needle and thread size suitably. If you want the straightest line of stitches you might go for sharp point needle, but if you want the stitching to go between the treads in the knit you go for ball point tip or micro tex, the last two is generally recommended for jersey. For jersey it often helps to lower the presserfoot tension, but not all models have this function.