at least it doesn't appear that they are skipping stitches. I think you are close, try sewing 3 layers of cotton, or a couple layers of denim. Just my 2c worth, but that material appears skimpy and the bobbin thread may not have much choice if the material is kind of shear. i may be wrong, probably am. The other points might be to look at using the same thread top and bottom, with a new standard needle. If you can stitch well under those circumstances, other materials and threads are just minor balancing of tension, needle dia, etc. IMO