I think it is also possibly because of the imperative verb form you used - montre is appropriate for tes, not for vos, which would need montrez-moi. In usage, I guess it is also unlikely you would use an imperative in this context for someone you needed to address formally - except maybe if you carried some official role/authority as well? Hopefully this is correct!

could it be your answer was marked wrong --not because of what you think -- but  because you didn't agree the verb with the pronoun properly?  you mixed the forms.  it should've been either montre-moi TES mains or montrEZ-moi vos mains.