I have this problem with button down shirts. I buy one that fits me perfectly and then all of a sudden months later its too small in the sleeves. I wash them very carefully. Only cold water, and air dry. This helps somewhat but the problem seems to re-occur still. I'll try the delicate cycle on the washer, but it's incredibly frustrating.

Make sure they are pre-shrunk cotton. It can still shrink, but far less.

Make sure the shirt is sized so that when you extend your arm laterally, the cuff approaches your knuckle.

If your idea of a fitting shirt happens to be that the cuff just covers a wrist watch when you extend your arm, then you have no spare headroom for shrinkage.