The problem is probably due to the difference in cast on and bind off tension. If the cast on edge is tighter than the bind off, bind off tighter (with more tension on the yarn) or use a smaller needle. If the opposite is true do the reverse, and bind off with a larger needle and ease tension on the yarn.
A bind off that I use when I need a stretchier bind off is the English bind off. Knit one, pass back to left needle, knit 2 together, pass stitch back to first needle and repeat across.
