custom mob heads dont stack plz help

i am trying to make a custom mob head plugin using following code but the heads doesnt stack plz help

public ItemStack getCustomSkull(String url) {

ItemStack head = new ItemStack(Material.PLAYER_HEAD);
if (url.isEmpty()) return head;

SkullMeta skullMeta = (SkullMeta) head.getItemMeta();
GameProfile profile = new GameProfile(UUID.randomUUID(), null);

profile.getProperties().put("textures", new Property("textures", url));

try {
Method mtd = skullMeta.getClass().getDeclaredMethod("setProfile", GameProfile.class);
mtd.setAccessible(true);
mtd.invoke(skullMeta, profile);
} catch (IllegalAccessException | InvocationTargetException | NoSuchMethodException ex) {
ex.printStackTrace();
}

head.setItemMeta(skullMeta);
return head;
}

 

I think player heads don't stack by default.