Most of the time stacking works as you'd expect it to, but twice now I've encountered a problem. A stack of 2 and a stack of 3 Silver should stack up on each other, shouldn't they? As well as 2 and 2 Uranium! But they didn't. Everything else stacked fine, Silver stacked fine a few other times even... Uranium at now point allowed for more than stacks of 2 though, now that I think about it. That can't be right....??