If there is no user- or technical error, you need to be connected to the new class/group via an invitation code.
Are you logged into the app with the correct email address that has access to the desired classes/groups?
Update the app and clear the cache by selecting "Clear cache and log out" to rule out any technical errors.
Ask the class/group admin for the invitation code for the new class/group – add it in the app via "Add Code". A FoxAdmin can also add you to a new class/group without providing you with the invitation code.