You have not fully unfurled the sail. There may be a wrinkle right at the spreader. Tighten the furler line and gently pull at the outhaul to free it. Don’t use an electric winch for this. When it is out and you begin to furl
1. Check boom at 90 degrees to mast
2. Keep gentle tension on the outhaul
3. If a wrinkle starts to develop at the mast, stop, pul it out and add a bit more outhaul tension.
If it does this all the time the sail may need to be recut or replaced as there could be too much belly due to age or stretching.