Over time the mount / demount heads (also called duckheads) on your tire changer will wear. To get the best results from your tire changer mount / demount heads replace them whenever they show wear or are broken. Note: mount / demount heads (also called duckheads).
There is also a trend towards converting from steel mount / demount heads to plastic ones. Many plastic heads ride closer than steel and reduces stress to beads when mounting and demounting low profile tires. Other pros of plastic heads are that they protect the tire and rim. Plastic heads aren’t suitable for all tire changers.