One of the following will be the problem:
One of the devices in the game doesn’t have the latest version of the app - please download the latest version of the app on every device.
The join-game code is being mistyped by a joining player – make sure to use capitals and lower case exactly as displayed.
The device which started the game (the host) is not waiting for the other players to join. The host must wait at the stage where the players appear behind their podiums, and until all joining players have a tick against their podium.