They're a very soft compound because they're not supposed to be used when the vehicle is moving, but they do need to provide lots of static friction. It's VERY easy to "cook" them, so the low speed is VERY important. (Don't ask me how I know this...)!

Great advice, thanks Malinois.

Sometimes the shoes do not contact the drum all around the shoe with "high" spots limiting the contact area, notably either the inside or outside edge of the shoe.
I usually chalk the shoe surface, put the drum on and with the handbrake on lightly, turn the drum a couple of rotations as it rubs the shoes. Then after releasing the handbrake and taking the drum off you will see if most of the chalk has rubbed off the shoes (good) or if only in some small patches (bad) which will be the high spots which you can sandpaper off a little.