Borderlands-Made in New Haven Glitched Achievement?

I have spent ages doing every single plot mission, side mission, claptrap rescue up to the point where i get to the salt flats. But the Made in New Haven achievement didn't unlock after i did them all. I even bought the official strategy guide before hand to tick off all the missions I'd done. I have since checked and double checked and I have done all listed missions so far but still no achievement. Will it eventually unlock? If so when? Could it be something to do with the disc? (coz there is a shiny line running around the edge for some reason). Please help, I don't want all my free time to have gone to waste.