Call me a noob or whatever but I can't figure out how to unlock weapon attachments or weapons, I know that they are rewards for completing challenges and their are 3 difficulty levels but i'm having trouble finding out how to change the difficulty.

I've completed each of the challenges once with the exception of the first which I have done twice and I haven't unlocked a thing apart from when I ranked up.

Any help is appreciated, if it is completely obvious and I am just oblivious to it then I apologise for my stupidity.

Make sure your in solo if not you wont unlock anything

When i did challenges for my attachments I did them solo and offline and they changed themselves. I just picked the same challenge again and it was the next star up. If you've tried that and still didn't work... I don't know.. Try deleting all your game data and start fresh and try again.

Ahh thanks guys, i've been running 2 player co-op.

I feel stupid for not knowing that so i'll just hide behind an excuse that it doesn't actually say anything about solo.