The congiuntivo passato is used when the action of the dependent clause took place (or was believed/hoped to take place) before the action of the main verb (the subjunctive trigger). Examples:
Credo che ieri Giulia non abbia votato per il mio partito.
I think Giulia yesterday didn’t vote for my party
Spero che tu non sia andato a casa da solo!
I hope you didn’t go home all alone!