I don’t do sales so have no experience with this. However I just went and had a look in your shop and the discount code only appears on your shop front so if someone lands on a listing they don’t necessarily know about it. If you use the shop announcement feature it will appear at the top of every listing so if people do land a listing they will see that you have an active discount code and might be tempted to use it. Also people might be waiting until after pay day to treat themselves, january can be quite tough budget wise.