I'm stuck, this is it, one of the players is at 37% and wont go any further, rest is 100%.
So what exaclty can one do when this happens?

This also addresses players that are offline from the game start, they just never loaded the game, should even the game start? Discuss.

Sounds like the issue was with your loading screen not theirs, since it has an internal timer of the max time allowed for everyone to load, if one player cant connect during that time, the game will start w/o that player. So you should ve just reloged yourself.