Nikon 24-70 is not taking sharp photos, why?

I'm very discouraged that it's been a year since I purchased this lens. Maybe 10 percent of the photos come out sharp but most of the time they are blurry or out of focus.

I have tried indoor, outdoor, day, night, speed light, different settings, the results are just disappointing.

I'm using Nikon D90.

Can someone tell me why?

Assuming that you have turned on the autofocus, that you have used a tripod to rule out camera shake, that the camera has chirped to confirm the autofocus has looked on, and you have experimented with other lenses to confirm that the camera works well with them,
well, sometime Nikon does make lemons, and very expensive ones, that do not autofocus accurately.

Back/front focusing issues with that individual lens perhaps?

Download a focusing chart like I did here for another lens

http://www.pbase.com/johnfr/image/55529218

Line up a series of objects like I did here maybe?
.
http://www.pbase.com/johnfr/image/49599175

Not focusing on the part of the subject you should be?

When you start using fast lenses (f/2.8 and faster) you need to re-learn how to use your camera… Specifically your focusing mode.

Especially at 70mm and f/2.8, the DoF will be a bit on the shallow side, so if your camera is not focusing on the proper point, the intended object may be blurry.

Next time you use it, see what point the D90 selects when you focus the lens. If it does not select your subject, it can end up being blurry.

The remedy - and what most photographers go to when using fast lenses (especially fast telephotos), is to go to spot focusing, which allows you to select which focus point you want the camera to focus on.

When I first went to the Nikon AF 80-200mm f/2.8 on my D90, I had the same problem with poor focusing until I discovered I had to re-learn how to focus the camera. When I went to spot focus, it fixed the problem.

While this may or may not fix your problem, rule it out by going to spot focus. The 24-70 is a fantastic lens, and I would not automatically say there's something wrong with it.

You need to show a sample image with the EXIF data intact. Too many things can cause a blurry image. It can be operator or equipment issues.

Could be several reasons but we can't say for sure without knowing how you are using the camera. Is it on Auto or Manual focus? What sort of exposure settings are you using?
Post a few photos here and we might be able to say more.