Ugh! Sounds like a frustrating experience.
Some other multiplayer games (made by other companies) are oh-so-easy to join.
But, as you've experienced, it is difficult to join a multiplayer HoI4 game.
One of our players in our multiplayer group has more than 4000 hours logged in playing HoI4...yesterday, he could not join for more than 40 minutes. A similar thing occurred last week. It is not easy to join a multiplayer game. Not easy at all. Not even for a group of players with a combined playing time of more than 30,000 hours.
All I can offer is a Podcat Prayer © :
"Oh Podcat, make the multiplayer game oh-so-easy to join so that players can enjoy the multiplayer experience, the game will be more fun, sales will increase, your year-end bonus will increase, and Fredrik Wester will be pleased."
(hmmm...don't want to be too irreverent).