The fire for the skewers should be medium, even medium-low, so that they cook slowly and don't get burnt. Rotate the skewers as often as possible and sprinkle from time to time with a little water or marinade, if the fire is too strong and makes flames.
Otherwise, the meat can quickly get too dry, or it can burn and stay uncooked in the middle.
Notice that we use a grill without the gridiron, the skewers rest on the walls. But they can also be put on the gridiron, if you don't have the right grill.