GTA Online - Cannot join private games when invited!

[Acky.]

Okay, so I've been trying to play Online in a private session with a couple of friends and I'm having trouble joining the session.

 

I will get an invite and the invite puts me into the Xbox Live party and says that I'm in the game, even when in a public match or in single player (I get no notifications).

 

Also, if I get sent an invite whilst not in the party, just for the game, it does the same.

 

I have also tried the method of going through;

Crews -> Friends Crews -> The Crew -> Friend -> Join Game, and I get the alert that says "The game you are trying to join is private..."

 

Is there any fix for this? My NAT-Type is Open and I was able to join friends' private sessions earlier on in the week.