This guide explains how to join a tournament even though, you clicked the “Join” button as soon as your atomic clock hits the appropriate time and the status remains “Checking in…”.
2. As soon as the “Join” option is available, smash that join button,
3. If you’re not a part of the tournament and the tournament is not full, immediately abort your request and repeat step  and  approximately 5 to 10 times.
4. WHILE your status is “Checking in…”. Hit the WINDOWS-Key and close Rocket-League with “Right-Click” -> “Close Window”
5. Start Rocket League again and join the Tournament
6. You are now part of the Tournament (if it is not full already)
This seems to be an obscure guide but I can ensure you, that it often works.
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