Hey there, Anna!
Karlie here from the Shopify Guru Team! The sidebar feature of the Brooklyn theme is meant to adjust depending on the viewers screen size, so it won't always show up on your website. However, because you are using Brooklyn (which is a Shopify-supported theme), I would be happy to escalate this issue up to our theme team for you to see if they can have the sidebar show up 100% of the time for your customers. I will send you an email and we can go from there!
I will be in touch soon via email!