Your leech is probably/possibly ''blown-out'' or permanently stretched.
Remedies:
1. If this is an costly sail, it can be ''rescued'' by applying a leech tape, a draw string and a
cleat to fasten the draw string. -- easiest but moderate cost.
2. Alternately the leech can be resewn along by ''tightening'' the joints of the panels near the leech. --- hardest and most cost
3. Very small, short length battens can be added - ''cheap and dirty'' but small cost.