Why aren't all metals included in the reactivity series? For example why isn't molybdenum included? Now we know that hydrogen displaces those metals from their oxides that are less reactive than iron. But how do I know whether hydrogen would also displace molybdenum if it's not there in the reactivity series?
That said, as @Karl mentions, reactivity series I've seen usually are incomplete, often leaving out transition metals with multiple valences such as Mo and W. To fill in the missing species, look at a good table of the electromotive series.
The omission of so many metals is downright shocking, revolting. One would think a current table would have included more metals; these reactivity series seem like works in transition.