In Minecraft there are several different types of item filters that can be used to separate items. If you don't know how to set them properly then things like shulker loaders can break. All these images are from CommandLeo in the Storage Tech discord.
The item filter on the left is a simple item filter but isn't overflow proof, so if you have multiple filters lined up next to each other and one gets filled it will break all the others. The design on the right is overflow proof.
Double speed loaders usually need all the containers pre-filling, this is how to fill them.