Strange Itemstack NBT behaviour

[Quarg]

I have a staff item that I want to have drain energy on usage, the current way I have it set up is that this function in the staff class is called via a tickhandler:

public float doDrawEnergy(ItemStack item, EntityPlayer player, float energy)
    {
        if(item.getTagCompound().getFloat("charge") > energy)
        {
            item.setTagInfo("charge", new NBTTagFloat("charge",item.getTagCompound().getFloat("charge") - energy));
            return energy;
        }
        else
        {
            float out = item.getTagCompound().getFloat("charge");
            item.setTagInfo("charge", new NBTTagFloat("charge",0));
            return out;
        }
    }

The problem is that the "charge" nbt value doesn't always change, specifically it only that the getFloat function only returns the latest set value after opening a GUI (ie: it only seems to upate the value after opening a GUI), and I have absoluetly no idea why or how this happens.

[SamTebbs33]

I'm doing the same kind of thing and I have no idea how to get ItemStack containers to save NBT data