Pigs, sheep, and cattle will often refuse to enter a conveyor restrainer due to the visual cliff effect. They can see that the conveyor restrainer is elevated. Old black conveyor belting works well. This makes the animal think that it has a solid floor to walk on. Entrance ramps on conveyor restrainers should be non slip and have cleats or ridges for traction.
Animals tend to back out and panic if they slip.
To induce animals to ride calmly in a conveyor restrainer install a solid piece of metal over the top of the restrainer. This metal top must be long enough to block the animal's vision until it's back feet are off the entrance ramp. If animals can see people before their feet are off the entrance ramp they are more likely to rear up. Experiment with pieces of cardboard before installing metal. Vision controls behaviour in livestock.