Maria, I have had a similar problem (with a different camera/lens). I have noted two possible causes. First is in low light or low contrast. It seems that especially with low contrast, it depends on having a focal length that includes enough variation in contrast in the auto focus region. Two ways to potentially handle this. Once you have found a focal length you can focus at, hold the shutter down 1/2 way down, then change the focal length of the lens. OR - change the AF focus method to, say, center weighting.
The other thing I have noticed is sometimes the battery indicator say everything is okay, but it is not sufficient for focusing under these low light/contrast conditions. Changing/recharging the battery has often resolved the problem.
Hope it helps.
Mike