Hello Why not "were we showing you our gardem?" Since pokazywać is imperfective.
This is an accepted answer (well, with 'garden' and not 'gardem'). However, "pokazywać" belongs to those verbs which work kinda strange between perfective and imperfective. Similar ones are "czytać/przeczytać" and "oglądać/obejrzeć".
The imperfective ones here kinda have some 'by any chance' component. So "pokazywaliśmy...?" is just "Have we by any chance shown you our garden? I don't remember if we did", while "Pokazaliśmy...?" suggests that we were planning to do so. That's at least how I feel. It's more obvious with the other pairs of verbs I mentioned: "Czytałeś Harry'ego Pottera?" asks if you by any chance ever read Harry Potter, "Przeczytałeś Harry'ego Pottera?" suggests that I knew that you were supposed to read it and I'm asking if you finished.