[1.7.10] ItemStack NBT not syncing

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I have an item (Snow Pouch) that should refill snowballs when thrown, but it seems like the NBT data isn't syncing. In game, the slot of snowballs in my inventory goes back and forth between 15 and 16 every other throw (should stay at 16). The Snow Pouch shows total snowballs in the tooltip, but the number doesn't appear to change unless I pick it up from my inventory and put it back. What am I doing wrong?

 

@SubscribeEvent
public void onEntityJoinWorld(EntityJoinWorldEvent event) {
if (event.entity instanceof EntitySnowball) {
	EntitySnowball snowball = (EntitySnowball)event.entity;
	if (!event.world.isRemote) {
		if (snowball.getThrower() instanceof EntityPlayer) {
			EntityPlayer player = (EntityPlayer)snowball.getThrower();
			ItemStack snowPouch = getItemInHotbar(player, ZeroItems.snowPouch, 0);
			if (snowPouch != null && snowPouch.stackTagCompound != null) {
				int snowballs = snowPouch.stackTagCompound.getInteger("Snowballs");
				if (snowballs > 0) {
					if (player.inventory.addItemStackToInventory(new ItemStack(Items.snowball))) {
						snowballs--;
						snowPouch.stackTagCompound.setInteger("Snowballs", snowballs);
					}
				}
			}
		}
	}
}
}

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