The phase detection autofocus on your D7000 works by calculating the disparity between an observed brightness pattern as seen by two AF sensors. These are effectively miniature sensors consisting of single rows of pixels.

In order to be able to find a reliable match and thus measure the distance there needs to be a strong variation in the brightness observed over the length of these strips, i.e. there must be a line feature or someother local detail.

The phase detection autofocus on your D7000 works by calculating the offset between an observed brightness pattern seen by a pair of miniature sensors, consisting of single rows of pixels, that make up each AF point.

In order to be able to find a reliable match and thus measure the offset there needs to be a strong variation in the brightness observed over the length of these strips, i.e. there must be a line feature or some other local detail that crosses the strip.

The areas you are focusing on simply don't have enough detail for the camera to find an unambiguous match. There are a couple of other cases when you can have detail and still not be able to focus.

Although displayed as a little box in the viewfinder these AF strip pairs have either horizontal or vertical orientation (or both in the case of a "cross type" AF point usually found in the centre only). Thus if the AF point is parallel to the edge there wont be any variation along the length of the strip.

Finally the boxes displayed are not always very well aligned with the actual AF sensors, so if you have a small detail surrounded by a plain surface it's possible to miss it and AF to fail even though the box in the viewfinder looks like it's right on it!