Only classes that members are able to book in to will show under the app's Classes section. To see all the classes that are on at your facility, direct them to the Schedule menu option. If a class is missing then that means that class cannot be booked online by the logged in member.
You can verify that the class is missing by checking your Member's Portal as that member. Because the Member's App interfaces directly with the Member's Portal, you can login as any of your members and see what will be available to them through the app. See the article, Login to the Member's Portal as a member for more details.
If a class is missing, it might not have been enabled for online booking. Navigate to the classes configuration page, Settings > Booking and Classes > Class Configuration, then select the class to . To be booked through the app the class will need to have Enable Booking Online checked. The Earliest and Latest Pre-Booking times can affect which classes will show too. E.g. by default classes can only be booked 2 weeks in advance, so the app will only show classes this far in to the future.
If the class has any benefit restrictions you'll need to double check that member's benefits. Navigate to the member's profile in GymMaster, then look under their Memberships tab. At the bottom of this tab there is list of all the benefits granted by their memberships. Check that there is a benefit for the class they're trying to book, and that it has enough balance remaining.