Sorry, I should of added more info. In bright daylight it still wont lock. If I'm just say at the local park and want to take a photo it wont lock focus, even when it's sunny and bright. The only way I can get it to lock is when I zoom it in to about 30mm+ It's really bugging me because I have to take photos of a charity day on Saturday and I'll only be able to get shots of single people instead of a whole group. I have tried to manual focus at 18 but can't seem to get it right, always comes out unfocused. I have two 18-70's, I thought my old one was broken because of this and picked up another one, now they both have the same problem. I also have a 70-300 and that locks perfectly. on both my D90 and D7000. I have it on AF-S with single point. I have also tried other focus settings but nothing works. I thought it could of been dirty sensors but looking at the zoomed in shots you can see they are clear and sharp.
I'm going to try my Sisters 18-105 tomorrow, hopefully that will narrow it down to the lenses or cameras. If that still doesn't work I'll have to take it it to a shop. Thanks for taking the time to think about it anyway.