The material was probably not cut on straight grain on that leg. It was cut more toward the bias. As you are sewing you are probably stretching the cuff so it puckers. Now that you have it pressed, run a basting stitch along the top of the hem--right through the hem and leg. Then go back and do your hemming. Remove the basting when you are finished. Be careful not to pull the material. If you have to leave a small gap on the turned up hem side between stitches. This will iron out later.

You don't say which is puckering, the hem or the leg area. The side seams might be a bit more tapered on the 2nd leg - that would make the hemmed edge shorter than the part you're trying to attach it to, or vice versa. If the leg area is puckering, you might be able to open the side seam enough to add the extra length you need, & if the hem is puckering, you can run a line of stitching along it & ease it in to fit.

HTH

Actually I'm not sure which was causing it.I do feel it was something to do with cutting them not straight perhaps.Anyway I took the hem out of the other leg and hemmed them by hand and had no problems.