Would you even feel the blows of a staff through your armor? We are not told that the monk character in Rogue One has superhuman strength, and it seems odd that he can topple stormtroopers with one swing. What good is a stormtrooper's armor if it doesn't even protect you from a staff?
Blunt force is effective against armour, especially when said armour is designed mostly to block against plasma or directed-energy weapons, like those most commonly seen in the Star Wars universe. Remember, the Ewoks used rocks to murder Stormtroopers back in Episode VI.
Basically, the force of a blow is transmitted through the hard material of the armour perfectly and still bruises the tissue. Even in the late mediaeval period people knew this and began to prefer the mace.
Think of it like this: you are an Imperial armour designer - what do you put in the armour, layers and layers of padding for the occasions when your troopers are so incompetent that they get attacked by old men with sticks, or some kind of fancy science plastic to dissipate blaster bolts, the thing that currently kills 99% of soldiers in the galaxy?
Stormtrooper armor was deliberately piece of cr*p, Emperor didn't want invulnerable minions
As we could see from this article, Stormtrooper armor gave some protection against elements, projectiles and blaster fire, but little to none against blunt and other melee weapons, not to mention lightsabers or Force.
Reason for that is simple: Stormtroopers were Palpatine's army. Palpatine eliminated Jedi by organizing the plot where their own troops fired at them. He was completely aware there was a greater danger for him and his rule in betrayal by his own servants, than to be killed by some outside enemy.
As such, he made sure that all of his minions, and all equipment they use, has some sort of flaw that could be used against them. So, Vader's armor is susceptible to Force lightning, Tie fighters have no shields, Death Star has exhaust port (Ok, maybe he didn't want that :) ) and stormtroopers are basically cannon fodder designed to kill and be killed.
Of course, when needed, Palpatine would provide better armor to his troops, especially his own personal guards. But these soldiers were far more loyal to him then regular ones.