Why does a t-shirt neckline stretch out and how do I stabilise it?

The neckline stretches when the rib is too long, the fabric is pulled while sewing or the shoulders are not stabilised. Apply stabilising tape on the shoulder seams, cut the rib at the right ratio, around 80–85% of the neckline, and sew without pulling. Choose a rib with elastane for recovery. After sewing, steam the neckline into shape. Check after washing, not only before. A well-built tee keeps its neckline after many washes, which is a quality sign buyers notice immediately in the shop.