Very simple error: my inventory on my container is not saving. Here is my TileEntity class, all is registered correctly, the inventory just won't save.

 

package com.happykiller.weightlimit.blocks.tileentites;

import com.happykiller.weightlimit.api.ICarryWeightModifier;

import net.minecraft.entity.player.EntityPlayer;
import net.minecraft.inventory.ISidedInventory;
import net.minecraft.item.Item;
import net.minecraft.item.ItemStack;
import net.minecraft.nbt.NBTTagCompound;
import net.minecraft.nbt.NBTTagList;
import net.minecraft.tileentity.TileEntity;
import net.minecraft.util.ChatComponentText;
import net.minecraft.util.EnumFacing;
import net.minecraft.util.IChatComponent;
import net.minecraft.util.ITickable;

public class TileEntityUpgradeStation extends TileEntity implements ITickable, ISidedInventory {

private final String name = "Upgrade Station";
private final String tagName = "WLUpgradeStation";

public static final int INV_SIZE = 3;
public final int SLOT_UPGRADE = 0;
public final int SLOT_MATERIAL = 1;
public final int SLOT_RESULT = 2;

ItemStack[] inventory = new ItemStack[iNV_SIZE];

public boolean inventoryChanged;

public int getSizeInventory() {
	return inventory.length;
}

public ItemStack getStackInSlot(int index) {
	return inventory[index];
}

public ItemStack decrStackSize(int slot, int amount) {
	ItemStack stack = getStackInSlot(slot);

	if(stack != null) {
		if(stack.stackSize > amount) {
			stack = stack.splitStack(amount);

			if(stack.stackSize == 0) {
				setInventorySlotContents(slot, null);
			}
		}else {
			setInventorySlotContents(slot, null);
		}

		this.markDirty();
	}

	return stack;
}

public ItemStack getStackInSlotOnClosing(int slot) {

	ItemStack stack = getStackInSlot(slot);

	if(stack != null) {
		setInventorySlotContents(slot, null);
	}

	return stack;
}

public void setInventorySlotContents(int index, ItemStack stack) {
	this.inventory[index] = stack;

	if(stack != null && stack.stackSize > this.getInventoryStackLimit()) {
		stack.stackSize = this.getInventoryStackLimit();
	}

	this.markDirty();
}

public int getInventoryStackLimit() {
	return 64;
}

public boolean isUseableByPlayer(EntityPlayer player) {
	return true;
}

public void openInventory(EntityPlayer player) {}

public void closeInventory(EntityPlayer player) {}

public boolean isItemValidForSlot(int index, ItemStack stack) {
	if(index == this.SLOT_UPGRADE && stack.getItem() instanceof ICarryWeightModifier)
		return true;
	else
		return false;
}

public void writeToNBT(NBTTagCompound tag) {
	NBTTagList tagList = new NBTTagList();

	for(int i = 0; i < this.getSizeInventory(); i++) {
		if(this.getStackInSlot(i) != null) {
			NBTTagCompound tag1 = new NBTTagCompound();

			tag1.setByte("Slot", (byte)i);
			this.getStackInSlot(i).writeToNBT(tag1);

			tagList.appendTag(tag1);
		}
	}

	tag.setTag(tagName, tagList);
}

public void readFromNBT(NBTTagCompound tag) {
	NBTTagList items = tag.getTagList(tagName, tag.getId());

	for(int i = 0; i < items.tagCount(); i++) {
		NBTTagCompound item = items.getCompoundTagAt(i);
            int j = item.getByte("Slot");
            ItemStack stack = ItemStack.loadItemStackFromNBT(item);
            
            if(stack != null) {
            	if(j >= 0 && j < this.inventory.length) {
            		this.inventory[j] = stack;
            	}
            }
	}
}

public void markDirty() {
	for(int i  = 0; i < this.getSizeInventory(); i++) {
		if(this.getStackInSlot(i) != null && this.getStackInSlot(i).stackSize == 0)
			this.setInventorySlotContents(i, null);
	}

	this.inventoryChanged = true;
}

public int getField(int id) {
	return 0;
}

public void setField(int id, int value) {}

public int getFieldCount() {
	return 0;
}

public void clear() {
	for(int i = 0; i < inventory.length; i++) {
		inventory[i] = null;
	}
}

public String getCommandSenderName() {
	return name;
}

public boolean hasCustomName() {
	return name.length() > 0;
}

public IChatComponent getDisplayName() {
	return new ChatComponentText(name);
}

public int[] getSlotsForFace(EnumFacing side) {
	return null;
}

public boolean canInsertItem(int index, ItemStack itemStackIn, EnumFacing direction) {
	return this.isItemValidForSlot(index, itemStackIn);
}

public boolean canExtractItem(int index, ItemStack stack, EnumFacing direction) {
	return false;
}



public void update() {
	if(getStackInSlot(SLOT_UPGRADE) != null) {
		ItemStack backpack = getStackInSlot(SLOT_UPGRADE);

		if(backpack != null && backpack.getItem() instanceof ICarryWeightModifier) {
			ICarryWeightModifier itemProps = (ICarryWeightModifier)backpack.getItem();
			int amount = itemProps.getAmount();
			Item material = itemProps.getMaterial();
			Item result = itemProps.getChild();

			if(getStackInSlot(this.SLOT_MATERIAL) != null) {
				ItemStack materialIn = getStackInSlot(this.SLOT_MATERIAL);

				if(materialIn.stackSize == amount) {//Do slots

					this.setInventorySlotContents(SLOT_RESULT, new ItemStack(result, 1, 0));
				}else {
					this.setInventorySlotContents(SLOT_RESULT, null);
				}
			}
		}
	}
}
}

Solved it! Forgot to call the super for write/read nbt.