When you sew in a sleeve, an adjustment needs to be made in the curves at the side seam ends of the armhole seam to allow more freedom of movement.  Two choices are:  1) Sew a second line of stitching in the seam allowance 1/4" away from the seam.  Then trim the excess seam allowance close to the second line of stitching from the side seam toward the shoulder for about 4" on each end of the seam.  2)  Clip into the seam allowance to about 1/8" away from the seam about every 3/4" for 4".