Etiquette for contacting a corresponding author?
Ask your advisor. In this context I think it's more important to show you have some forethought to realize the situation could be potentially tricky and that you know when to ask for advice, versus showing your initiative/independence. You're a second year graduate student. 

There are several issues that your advisor will know more about than you regarding your relationship with this other lab, in terms of publication strategy, any personal contact they already have, avoiding your request sounding like an accusation or making you look foolish, etc. Your advisor might also know about the reputation of the other author: maybe this is someone who would be responsive to a student email, maybe it's someone who would ignore you and your advisor needs to intervene on your behalf. Your advisor can also give you recommendations on your own work and help you discover if there are other causes for your inability to replicate results so far, and save you both from some embarrassment if the problem is your own.