Use a ballpoint needle with knits. A mercerized cotton thread does not have as much stretch as synthetic threads. Using woolly nylon in the bobbin to sew a plain seam makes it more elastic. When filling the bobbin, use a slow speed. When polyester-cotton thread is wound at a high speed, the threads stretch, causing puckered seams in the finished garment.
Elastics differ in their stretch and recovery characteristics.
A braided elastic narrows when stretched and can lose stretch and recovery if it is pierced when stitched. Knitted and woven elastics retain their original width when stretched. There are special elastics for cycling pants, swimwear, lingerie and sweatpants. Select a seam with enough stretch for the fabric. Use a plain seam for stable and moderate-stretch knits.