This happens because this game is trash, and no one ran any QA or stress testing on it before release. 

One possible reason is that there is only one generic error message when you can't join a game.  So if the game is full or was closed before it was removed from your custom game list it will give you the generic message and not a specific one to tell you what actually went wrong.  Since the refresh rate is so terrible though, you can't refresh fast enough to get the latest data on the available games.

Another reason this game is terrible is that everyone that joins a multiplayer game has to be able to connect to each other player, so everyone has to have ports 6100 through 6200 open.  Though, if someone without those ports open tries to join the game, it locks them out with an NAT failure, but everyone who is in the game or tries to join the game is still trying to connect to that one player, even though he's not there.  This could also hinder you from joining a game, but I'm not sure since there is only one error message.

So thats why its so hard to join a freeking game. I keep getting nat failures and failed to cennect to game msg's. I havent even been able to play a pantheon game yet wich is why i bought this stupid game. I think its really fun but why do you have to shoot yourself by making the matchmaking system so lame?

I constantly see 3-4 people in the connecting screen who isn't connecting or connected to anyone. FAIL FAIL FAIL.

Im really close to demanding my money back for a product that dont work at all as it should. Ffs i cant even play a single player campain to kill the time untill you fix this shit. Why? Becourse there isn't one.

Btw the game told me to update in order to play online. But i couldn't when impulse launched. But 10 min after i was able to log on so i got no idea if my game is updated or not.

What version is the updatet one? v1.00.0093?