This afternoon I took a scrap of denim put pale yellow denim thread in the top and bright orange denim thread in the bobbin. I used a universal Schmetz needle, then threaded up the Singer 15-91 to give it another try. The first thing I noticed was the orange denim thread was very tight in the bobbin casing so I loosened the bobbin casing screw to lower the tension on the bobbin thread. The top tension was set at 4, then I adjusted it to 3 and 3.5.
One of the things I love about sewing is that it teaches me patience. Sewing like everything else is a skill that requires practice to master. Hope this post was helpful.