You can continue to cook the yams.

Yam cooking time varies depending on the size and variety of yams. If the yams are still crispy after half an hour of cooking, it may be because the yams are larger or the varieties are harder. In order to make the yam softer, you can continue to cook it. Continue to cook the yam until it becomes soft and cooked. In addition, you can try adding some vinegar or lemon juice when cooking yams, which can help the yams soften faster.