All patterns have ease built into the seam allowances.Are you matching up your notches,and your shoulder seam to shoulder seam and underarm to the underarm seam.If you are doing these things,then you shouldn't be having that problem.Also on wovens,ie cottons etc you may need to run a basting line of stitches on the sleeve to help ease it in.On knit,pins should be enough.Make sure to read your directions on your pattern,especially if you are fairly new to sewing.
HTH
Kathi

carol
perhaps it isn't the armhole, but the measurement across your back that needs increased. I've had this happen. I had to add to the fold line on the upper back pattern piece. You might just need a little more ease across your back.
good luck!