There is text that says like "Deidra Moonmayden rolls Need 50", "Soandso rolls Need 20", or greed, or something like that. Have not seen mention of it yet, but it would go a long ways toward explaining. I assume you don't know what that text said -- just that you don't have the item now. In that case, it's mostly impossible to actually know what happened. Some possibilities were mentioned already (left instance before roll finished, clicked Greed by accident, bug, etc).