Originally Posted by Trevtassie

Somehow you've managed to cross thread the bolt, by tightening the bolt when it's not straight. You should be able to screw the bolt nearly all the way in with your fingers. Anyway, couple of things to try. Using something pointy to clean up the thread by working from the inside out. Using a M8 x 1mm thread tap to clean up the thread. Getting a new bottom bracket.