Do you mean by it saying „Unable to find a server” for the map?
Well, that's something because CubeCraft only have limited server allocated to a specific game. The way you should do it, is to wait in the map selection menu, wait all the way until the currently groups of people are entering their game, so you click the map you want to play at that exact moment. Because there is no map available for players to join, the server has to open a new map for you, and it will open the map you choose.
Well, if you are angry and wonder why they have to do something like this, imagine a group of malicious players, each open a different BungeeCord server (map) on their own. That will cost the CubeCraft having a lot of BungeeCord servers open for them, but actually they won't have enough players to start the game. They will occupy the valuable server resouces, but not actually playing games. CubeCraft has this function in place to prevent that kinds of malicious incidents, but I understand it is really a little bit annoying sometimes.