It's inevitable that this will happen to a certain extent. That's OK! If it happens while cooking and turning the kebabs, just let them cook on the side, then thread them back on the skewers before serving if you can. To stop it falling apart when threading it on the skewers, try using roughly 1-inch sized cubes (so neither too big nor too small) and pinching the cubes together from 2 sides as you thread it on.