As it is explained in the article about Visibility which can be found with this link, you can either make a guide hidden, visible in all pages, or just in specific pages. Hence, the guide might not be appearing on the Guide List in every page of the website. You should therefore modify your guide’s options on its visibility. You can learn more here.
If you are using Segmentation feature, you can also make a guide appear or not on the guides list by narrowing and distinguishing the groups of people you actually want to access a certain guide. Therefore, if the user visiting the site does not correspond to the audience you’re targeting with a certain guide, it will not appear. For it to appear you have to adjust your segmentation preferences.
Guide on Draft Mode
The guide might simply still be on Draft Mode, thus not visible in the Guide List. You have to make the guide public for it to appear.
A guide might also not be visible because the person does not have permission to see it. For instance, as an administrator, you have the power to make a guide visible or not, which means that while you can see it, others cannot. To make the guide visible you have to send an invitation to the new user you want to make the guide accessible to. You can do this through your Backoffice. If you want a detailed walkthrough you can check the following link here.
You might also have associated a guide with a certain category in your Backoffice. If this category in your Backoffice is hidden or in draft, so will the guide itself, and consequently, it will not be shown in the Guide List.
According to your Device Settings options, a guide might just not be visible on certain devices. You just need to modify those settings for it to appear. See here how to set up and check device options.