Origins - Updating items in Toolset causing them to disappear?

I'm trying to figure this one out: I recently tried using the toolset to update a few items from the vanilla game.  Change a few looks, swap a few stats, etc.  I went about it by basically creating an item in the Toolset with the same name and filename as the original, then copied them into my overrides directory.  I'm able to obtain the updated items in game, but when I re-load the game, the items disappear.  If the item in question was in my inventory, it simply vanishes (freeing the space in the inventory that it was taking up); if it was equipped, it's also gone, although any changes it made to stats remains on the character.  This isn't happening with wholly unique items that I've created, only attempts at modifying existing items.  I've used other mods, such as Improved Atmosphere, that adjust "vanilla" items, so I know it should be possible to do what I'm trying to do, but I'm not sure if there's something I'm not factoring in, or something I'm just doing wrong.