seams popping

[grooveslut.livejournal.com]

For Christmas I made my boyfriend some shirts. I sewed the seams and serged everything. Problem is he already has popped a couple of seams. They have not broken through the serged threads but through the normal straight line sewing machine stitches. I used Guttermann thread on the sewing machine (and factory left overs for the serger). I thought Guttermann was good thread...? Why are they breaking? there is no way for me to use cone threads (the factory left overs) on my sewing machine and I don't know what else to do. I want to make him some more shirts but I don't want to keep having to make repairs.

edit: a few people have suggested that I am using the wrong stitch. Apparently for stretchier fabrics I should be using a zigzag of some sort. Now that it has been mentioned to me I seem to remember reading this somewhere as well. I am going to try this next time. If anyone has any other suggestions please feel free to leave them also :o)