If you are in an invite only session that you are hosting it won't work you have to join a random session or have a friend host an invite only session for you.

I was doing it in a public lobby and it has even worked once, but now i cant get it to work again as i just dont have enough time to press start before it takes me to the xbox games store

Type in 27.27.27.27 and flood that. If that doesn't seem to work go to test connection and find out what ip your Xbox is, it should look like 192.169.(a number).(a number) then put that in your flood ip box. Also you are supposed to leave it on during the while thing not just half way or a little. Also I recommend using udp unicorn I use that.

I couldnt get it to work for days, then I started using a little timing method to get timing right and seem to have gotten it 6/10 tries since. Maybe start up a download or two on uTorrent and go to story mode to start. Once there Go to join ONline. When loading online right as the sequence is starting to drop down form the clouds hit Start on UDP flooder. Then as son as you get into the game press start and go to gamestore. If it fails just hit B to back out and try again with the joining sequence after leaving store when it starts to drop back down again