If the drums aren't turned smooth and have grooves or ridges, that can happen.
Say the new shoes were 1/8" wider than the old ones. The edges of the lining will be rubbing on the sides of the worn area before the face of the shoe contacts the face of the drum. You have to wear all of the edges off before the shoe will seat.
The same with ridges and grooves. You are grinding the shoes to fit the drums instead of machining the drums to take new shoes.
