Please check whether you have associated any Home Screen Layout policies to the device. Navigate to the Policies tab and click on the policy associated to the device. Go to iOS-> Configurations-> Home Screen Layout and you will be able to see if any configurations have been made.
If any Home Screen Layout configurations have been pushed to a device, users might not be able to change the arrangement of icons. In order to permit the users to change the arrangement of icons, the policy/configurations will have to be removed from the device.