Do you have a photo?
How visible? Small seam or large gap?
Which sts/line is visible? All sts? Picked up sts only? Knitted off yarn sts only?
I hate to say it but, this may just be the way it is.
However, it may be that you twisted the sts when you picked them up or knitted them off the yarn thus creating a row of twisted (more visible) sts.
Or, your tension could have changed between the body and the sleeve (especially if one was knit flat and the other in the round). Check your gauge.