Originally Posted by dakotamaid
QuiltswithConvicts, Try a tiny zigzag or a stretch stitch on your poly slacks. The stretch in them is what making your seam pop open. You need a stretch stitch to stretch with the fabric. JMO. :)
Dakota maid beat me to it. A double knit requires different stitching. If you use a straight stitch, the fabric will stretch, but not the stitches, so they break. A narrow zig zag will allow the seam to have a bit of 'give'.