Is there anything in the English section to say that the answer isn't pluralised?
I'm not sure what you mean exactly.
But "You are showing your book" could be speaking to one person or to several people.
Accordingly, Δείχνεις το βιβλίο σου and Δείχνετε το βιβλίο σας both make sense as a translation and should both be accepted.
Yes, both are accepted.
I've just had δειχνετε rejected!
What was the entire sentence that you wrote?
Thanks got it Εσύ δείχνεις το βιβλίο σου
Εσείς δείχνεις το βιβλίο σου got marked wrong
Yes, because the subject εσείς does not match the verb δείχνεις or the possessive σου.
Εσείς is plural or formal, δείχνεις and σου are informal singular.
Why isn't βιβλίο accusitve case here?
"το βιβλίο" is Accusative as well as Nominative.