The cloud says "Reconnecting"
The connection to the server dropped — almost always the school wifi rather than anything in the app. It reconnects on its own and picks up any answers sent while it was away. Nothing is lost.
Words are missing from the cloud
Check three things, in this order. Is the cloud paused? Is the update speed set to something slow? Is there a message under the cloud saying a word would not fit — if so, lower the largest text size in the Appearance tool.
If a specific pupil's word is missing, look in the Responses tool. If it is not there either, they have not sent it.
A pupil cannot sign in
They will see a message saying their address is not registered. Either their address, or their school's domain, has to be added by an administrator — see Signing in. If you also used the Access tool to restrict this activity, check that list too.
A cloud save failed
The message says which of the four likely causes it was: no space left, permission withdrawn, the service busy, or the connection lost. Signing out and back in fixes the permission case; the others are worth simply trying again.
If automatic saving turns itself off, it is because three attempts in a row failed. Save manually once to start it again.
Something looks wrong after an update
Reload the page once with Ctrl+Shift+R (Cmd+Shift+R on a Mac). That fetches everything fresh rather than from the browser's own store.