forstraight stitch i use 3 for tention on my machine i have 1-6 as a tension option. make sure you ahve wound the cotton onto the bobbin tightly. make sure you are putting the bobbin in corectly and its not bouncing around. Also check the bobbin is threaded correctly and stitch lenght is correct. Check you have the right settings and presser foot for the stitch u are doing. Also the bobin will normally go one way and the cotton will thread threw or catch on some thing check the manual or correct bobbin placement and threading.