Hi Angie,
This means that that particular session is not available to the registration type that is being assigned to the guest.
First, you'll need to check which registration type guests are receiving by going to Website & Registration > Registration > Registration Settings > Click on the Guests tab > Under Basic Settings, it'll be under "Guests can have registration types," and you'll see the registration types listed.
If you do not have this enabled, that means the guest is receiving "No Registration Type" as their registration type.
Now that you know which registration type is being assigned to guests, open the session that is causing issues (Event Details > Agenda Items > Sessions > Click on the session). Under the Availability section, you'll see "Limit which registration types can attend this session." Make sure you check off the guest's registration type here, then click Save.
We actually have a troubleshooting article specifically on this topic: Why are some sessions and items not visible during registration? (
https://app.cvent.com/Subscribers/Account/default.aspx?page=BE132BFF-ADD3-41F2-BCC9-4DBCE592E47E&targetPath=apex/CommunityArticle?id=000018004&Lang=en_US&searchTerm=limit%20session)
Best,
Julia