Hi! Are you trying to throw out iron/gold armour? Those items do not drop and just erase from your inventory, if this is happening with diamond armour, diamond swords, bows, pots, etc please sumbit a bug report! I would also recommend maybe trying to /trade in the meantime if its disappearing, as that wouldn't involve you throwing/dropping anything wrong rather putting straight into the person you traded withs inventory.
Like Tomato said above, gold/iron armour will disappear when you try to drop it. This is to prevent lag(?) I'm pretty sure. If you're talking about diamond gear then that must be a bug so please sumbit a bug report here on the forums.